You need to lawyer up. I know you don't want to but you need to get ahead of the ball. 
And keep posting!!!! We want to support you so these two don't win. 

I really feel bad how it is coming from all directions, the stress of SS and his BM, your other SS that lives at your home, and now your ex DH trying to manipulate your DD into living with him so he gets what he wants.

Can I ask how long your ex DH and SM have been together? To me it sounded like they have been together quite a while from what you had posted, but the way their behavior has been makes it sound like they are newly together. I guess the new baby is what is changing all this with them?

You have been more than considerate and flexible, but don't sit quietly on this one just to keep things amicable. I agree with Gimlet, I think it is time to talk to a lawyer. I do not think he is going to give up on trying to get CS reduced and is going to just look for another angle to make this happen until he gets what he wants.

I said it yesterday and I will say it today too as it is still true, your ex DH is acting like a child throwing a temper tantrum and is now basically trying to ignore you by pushing the matters off to his wife to handle. Whether that is by his want or SM's want it doesn't matter, it is unacceptable and it is clear they cannot handle this without having a lawyer or court involved.

I think you need to sit down with your daughter and explain reality versus the fantasy her dad is telling her. I also think you need to listen to her if AFTER you talk to her and she spends more time with her dad that she decides she wants to still live with him. 

My brother wanted DESPERATELY to live with my dad in spite of my mom being a good parent (at the time). It had nothing to do with her and everything to do with his preference in living situation. With my mom, he lived with two very loud, squabbling sisters. With dad, there would be quiet, and my brother wanted the quiet. 

I know right now that this is your ex and his wife trying to PAS your daughter. You combat that with truth. But, if she has solid reasons why she may want to live with her dad, you should consider letting her go LATER. Not sure when her birthday is, but you could tell her that she can decide at 14, but what she decides at 14 is where she stays until 18.

As for your ex, I'd give him one warning after you talk to DD that if he doesn't stop filling DD's head with promises he can't commit to that you'll be following the CO to the letter and seeking a readjustment to CS. Tell him that if he really wants DD to live with him, then he and you alone can schedule a time to chat about it together, either with or without a mediator. But using DD as a vehicle for communication and using her to "persuade" you stops NOW.

If he TRULY wanted what was best for DD, then he would be talking WITH YOU about it, not giving DD false hope. Having just gone through this with DH and ET, there is a right way and a wrong way to approach an actual custody switch. Going through the child isn't the way to do it. Talking to a child and asking their preferences, talking about reality versus fantasy, and setting realistic expectations for the long haul is right. What your XH is doing is wrong. IF his intentions are pure (BIG if that I wouldn't bet money on), then you're giving him the opportunity to address it with YOU, the parent. If they aren't, you'll have to address it via the courts.

This is all hitting at a bad time, and your anxiety is through the roof. I don't have much advice on that front, but know that you're not wrong in thinking this smells fishy. Combat the fantasy with the reality, address your XH directly, and take it to court if that's necessary. You're not there yet, though, and you won't be the bad guy if it gets to that point.

"Your dad wants to stay home, not work and not pay CS "

And use you for baby care.

Heck, he might even turn her into a full-blown Cinderella and get her to do all the cleaning too. 

I think you need to go into beast mode tbh.

To your EX.

Let me be clear.  I will not coordinate or discuss issues pertaining to our children with your wife.period.  You will communicate with me directly.  I will neither respond, nor acknowledge any communication from "that woman".  Your wife, your new child.. they should have zero impact on issues related to our children.  If you prefer that we communicate soley through legal representatives.. that can be arranged.

Also,  You will cease discussing potential changes in custody regarding our children.  You have no business "bribing" our daughter and setting her up for disappointment.  We have NOT agreed to any changes and there will be no changes in custody if I have anything to do with it.  All you are doing is causing your children to be confused and hurt by these conflicting messages.  Any attempt to discuss this with THEM will be considered an attempt at parental alienation and will be dealt with severely in a courtroom setting.  Consider yourself put on notice.

To your daughter.

I'm sorry that your dad has discussed things with you that have not been agreed to.  Since he has chosen to broach adult subjects with you.. he must feel you are old enough to hear more details about custody issues.  So, I will give you some further perspective.  Unfortunately, his motivation for changes appears to be driven by his desire to stay home with his new child and reduce the amount of support he pays for you and your brother.  Unfortunately, he doesn't have sole decision making on your schooling.  I would not be inclined to agree to home schooling and I think that you would agree that being isolated from a normal social life would probably not be as fun as he may be making it seem.  The bottom line is that he is promising something that in reality he can't guaranty.  And..Travel ball is something that you could pursue HERE too... if he is a proponent of supporting that.. he could certainly pay for that to happen now instead of asking me to get less financial support from him.  I was under the impression you were considering not doing softball in the future.. but if you want to pursue travel ball.. and your dad is in for paying for it.. we might be able to see how that would work here.  I know he is trying to make it sound super attractive for you to make a move... and I don't doubt that he loves you and likes the time you spend together, but he has no business discussing changes in custody with you that have not been agreed to.  And, you are old enough to know that the  motivation for these changes is that he wants to stay home with his new baby... and that you moving there would get him out of some financial obligations to me for your care.... Homeschooling won't be an option there..so what he is trying to offer you is not reality as it could play out.  I'm sorry that I have had to explain these things to you... I believe that adult issues should remain discussions with adults.. but I do also need to make sure that you are getting a more full and balanced perspective on what you are hearing from him.

Your ex wants to take BD out of school and home school her...why? Teenage years are when they should be with their peers not isolated at home with dad.
Given your daughters medical history will your ex be capable of dealing with that on top of being a stay at home dad caring for a toddler. I don't think it would be conducive for a toddler to spend all day in the ER while his half sister is being treated for a debilitating migraine.
The childish thinking that he doesn't have to deal with you because he hasn't had to in the past due to work and the not wanting to continue to pay CS for his children.
All of this presented to a lawyer is not going to bode well for him or paint a very good picture of him, even if he is a retired military captain/admiral/etc.

This makes me soooo sad to hear.  I thought things were smoother on that side!!!!

I'm glad you're sending the letter via your attorney!

To be a stay at home dad for his new baby. He needs to stop paying CS   
This is the point to all of this. He a real winner.    I must say.

   Home schooling your DD plays into this HOW ? 
He wants to take away your DD time in high school. Her time to thing what needed in life. Group working together. Being a Team as in business,  sports clubs ect. So he doesn't have to work.   

this lazy guy is going to be a great teacher 

I didn't have time to read all the responses.  I'd block SM from contacting you, and start a Google Calendar and share Read privileges with him only.  If he wants to give SM access, that's up to him.  Then only answer him, and ignore anything that doesn't need to be answered.

Custody battles cost $10-30k or more.  I doubt he has the funds for that.

ETA:  It's not going to look good for him that this all started with his requesting a CS reduction, and saying he doesn't want to work.  I hope you have all that crap in writing.  Start keeping a journal of all the promisess he's been making to DD.

EATA:  Don't reduce his parenting time, a judge would see that as vindictive.  And don't threaten him with court.  If you need to take him to court, just do it.  If he takes you to court, his CS will be reviewed and increased.  That should be a deterrent.

Co parenting is between two parents. Custodial issues are between two parents.

There is nothing in CO about dad's wife or mom's husband having custody or authority in parenting. Nothing. Zero.

So why dads or moms SOs would demand to be the ones to coordinate scheduling and custodial arrangements? I really don't understand.

So parents shouldn't coordinate anything and delegate to their spouses? By this logic OP's DH and SM should be making all arrangements about OP's kids and actual parents should step back. It's weird.

I get it some men are incapable and SMs step in but it's not something required. OP did not have children with ex's wife. SM isn't a parent in this equation. 

I'm confused, a judge most of the time WILL require the parents to communicate about their children.  Her communication isn't excessive, demanding or constant.  If they need to discuss something as big as custody change, or a change in visitation time that would be required by the courts for the parents to communicate, not put the SM in the middle.

Nothing bad has been said about the SM- the OP isn't making her the villain, she simply would rather communicate about big decisions like custody changes with the actual parent that says he wants to have her in his custody.  Makes perfect sense.  

Wellllll, about communication. I know a few cases where BM's have been told to send all important updates to ex's via mail, as in United States Postal Service. Lets face it some divorced parents don't talk at all.

Granted I am really coming late to this. Most of you know my xdh and I have 50 50 shared residential, NO support exchanged. I worked, he worked. He carried insurance.  Why would I take money from him for child support.  Our situation was not like some bm's where dad have zero time with minor kids,

. Anyway...Justmakingthebest...If I were in your shoes, I would 'consider'---just 'consider' this:

Let her go live with Dad. It is natural for her to want to go. Maybe you never thought this day would come??? I,  on the other hand,  am not surprised. Your x isn't a bad dad, or a drunk or a drug addict. Thank your lucky star's (((HUGS))))

 I would Keep her with you until August so you can have the majority of summer with her..

You could have a new court order written UP that clearly states IF daughters grades drop below, you pick the grade that is in her range,  she will be returned to YOU without issue at the end of the current school semester per Cali school district. Your x would have to agree with that reasonable entry.

I must admitt I am not a fan of homeschooling myself, but I am aware of success stories.

Flip the current visitation schedule since it seems to work ok.  You would have all summers, and all holidays.  You pay flight back to your airport, dad pays flight back to his.

You know your x has good medical insurance so that is not an issue, right?

Everyone on here brings to the table their own experiences. Mine turned out very good. It was not easy to get there but I am so glad we did.

I know you are upset, and I can tell it hurts too. I also know my consideration is nothing compared to what the majority wrote. It's just something to think about.

Edit to add....I tend to agree,  this is between xdh and you.

.

Have you gone online to calculate the CS for the son you have with you, giving he was underpaying by $400 a month?  If your calculations keep is a wash (same as you receive but he has full custody of DD) I'll bet anything he will back off from the deal so quick your head will spin. 

There will be some difficult days and weeks ahead. It's very hard.

Oh I hear ya, ^^^ there was a point I also thought, Hmmm jerk doesn't want to pay me cs so THAT is why he wants 50 50 .

But I could not over look he was their dad too...sure we were divorced but they love him as much as they love me.  I was on that fence Jerk, good dad, JERK JERK , good dad. Ahole doesn't want to give me money, good dad.  I knew I could win full custody. But that would hurt the kids.

Anyway this is not about me Justmakingthebest,,if you are reading this...I am so sorry you are having to go through this. It's a lot.

I know you will make the best decision for your situation.

I find myself musing over the fact that he thinks he'd be getting a good money deal by taking his daughter... This man clearly has no idea how much a teenage girl can cost. 

At one time, my husband was so wanting to have his son that he agreed to continue paying full CS if she would allow his son to live with us.  She happily agreed when she had a new boyfriend who didn't want a kid around but then yanked him back when she was alone and lonely and "needed him".  I told DH at the time to get it all legally changed, but he believed they would keep things cordial.

If your XH wanted your daughter for those reasons, I would consider it.  But he doesn't.

Also, I find it awfully confusing that she should both quit playing ball and join a travel ball team.  I think your attorney should take that as he is willing to foot the costs of paying for travel ball and ask to have that included in your new CS agreement requests.  You can always negotiate that away, but it sure seems like Dad is pro-travel ball even to the point of paying for it.