Oof. That's brutal. And also normal when you are asserting yourself for the first time. But if you back down, that's also teaching her/them they can treat you that way. That doesn't mean every situation needs to be high conflict, but it is absolutely and okay for you to assert reasonable boundaries and submit preferences when it comes to a schedule that also affects you. That getting you on a leash comment is wayyyyyy out of bounds. 
 

a therapist once told me every interaction is a Chance To set or reinforce a boundary. You are allowed to have them! And you're allowed ro enforce them! Also look into BIFF (brief informative friendly and firm) responses. It's a great way to manage high conflict situations, as is gray rocking. 
 

i know it feels icky to stand up for yourself at first but it Has gotten easier with practice for me!

I am not hearing much about your SO.  What does he think?  Sorry, I didnt see your prior post.  Does he agree with you?  I'm getting the feeling it's you against SO and BM.  Ideally, it would be you and DH against BM.

When I went to counseling, my counselor advised (beside assertiveness) more one-on-one time with DH.  Looking back, I see that helped him align more with me.  We also had a high conflict BM who used his fear of losing contact with the kids to get her way.

Good luck.

Don't pay too much attention to her comments about the kids disliking being around you. She was clearly on the attack and looking for blood. Just because BM says so doesn't make it true. You are allowed to have boundaries just like you're allowed to have feelings and needs. You're not the maid or the nanny. You're a person who deserves to be fully heard in your family. 
In my personal experience, DH has always  handled BM. We would decide things together and then he would deal with actually telling her what we were and were not willing to do.

As for the court threats, I know it's scary. But if BM sees all she has to do is mention court and you'll dance to her tune, life will be hell. The fact is if you have a HCBM you will be back in court eventually. Learn how to cover your butt, stay calm and friendly but firm when dealing with her. You don't have to say yes to everything but you should speak like the judge is listening. Then when/if you go to court you won't have given BM any ammo. 

Keep being assertive. What you need to figure out is if you're being assertive with the right person.  If you're asking BM to watch the skids during your SO's time, then you're likely talking to the wrong person.  It's up to your SO to make arrangements for child care during his time,  and if you don't want to watch them,  he needs to make other arrangements if he can't do it himself.  If, on the other hand,  you're expected to watch the skids during BM's time,  you need to be VERY assertive with both of them. 

Now that you've finally asserted yourself, don't back down. Stick to the boundaries you set, and let your SO handle BM. He should be backing you up 100% with BM, and if he's not, then he's a big part of your problem. 

Absolutely good comes from being assertive in the blended family world. You don't get walked on or taken advantage of.

What could be better than that?.. Other than a blended family opposition who is reasonable.

Yea... that happens.  But not often.

And try not to get upset, if you can keep it keep it simple in your mind. Oh needs to deal with his children on his time, and bm does on her time. It's not your job to be nanny or helper. If they can't do that they need to get grandma or a childminder to do it unless you specifically want to. If you want something to change such as the Weekend situation just make yourself totally unavailable until oh does change it, for example just go out with your kids and leave them to it.

It's not uncommon for bms to act this way when they're not calling all the shots and still controlling everyone. Try your best to ignore and make plans for your life. 

What you said was spot on and she didn't like it- that is all that is. HCBM's will go on the attack when someone calls them on their BS. They aren't used to having anything but their way.

There are lots of things you can do to help the parents get on track with being actually parents. For example, if it isn't your SO's time and  BM wants you to take the kids: "Sure, I charge $10/hr per kid. Pick up no later than 6pm. Dinner not included." 

SO wants to go do his thing while the kids are with you "Umm, hun, I think you are forgetting something. I am not your babysitter and this is YOUR time with YOUR kids, not mine. You need to take them with you."

They team up and say that you need to watch the kids "Nope, already have plans and can't take them with me."

Might I point out that, since BM says the skids hate your house, why would they want to be there every weekend anyway? That's bananas logic but use it to your advantage.

"I'm sorry those times don't work for me, I have x,y, and z to deal with...SO and BM I will leave you to figure this out."
Keep re-stating....those times don't work for me! Either your SO will have to start stepping up or BM will...these are their children! Should there be a medical emergency cannot take them to the hospital and sign off on anything...only their parents can. If you left tomorrow they would have to find other people to watch their children...stop being a free babysitter.

If someone threatened to put me on a leash, I'd never do a single favor for them again.

Honestly, you should be assertive.. but not with his EX.  that is his responsibility and it should be his obligation to deal with her,...not yours.  REally not much good can come of it other than making a tenuous relationship worse.

If the time you are talking about is HIS custody time.. it's HIS responsibility to ensure that he has childcare.. he can hire someone.. he can ask you to watch them.. OR he can ask his EX if she can or wants to take them.  But the bottom line is that in the realm of being assertive. HE is the one you should be assertive with over custody and child care on his time.  It's his responsibility.. not yours.. and honestly.. not his Ex's either really if that is according to a custody CO.. it's his problem to figure out.  If his solution is YOU?  then YOU need to push back at him.. not with his EX.. this isn't her responsibilty.

Everything that everyone has posted.... ^^^^^^^^^^^^^^^ every single word.

I would be stashing as much money as I can...just in case you need to find your own place.

Have you heard of restorative practice? I think in situations like yours, its perfect! Your 3 main points are: what has happened - how it's effected you and what you want to happen. 

But it's all about you, not them. Dont use the word 'you'. It's all I,I,I! 

For example 

I'm tired from doing so much and not getting any help from anyone else. 

I feel like i'm alone

I feel hurt by this situation

I would like some support with the children

See if it helps x it's all about you, no1 else x 

Is there a clear custody schedule for the kids? Like, a legal custody schedule with a CO? If not, there should be and it should be followed. If there's not a legal CO or if it's not followed, the waters get muddied IMO. You start "owing" each other and it gets messy. Stand up to BM when it comes to watching them on her custody time. And when it's DH's, stand up to him.

Also, i read your last blog. It's not fair for you guys to have the skids every weekend. Fight for a schedule that works for you, especially if you are expected to be their nanny.

Calm, cool and assertive - always. You failed in this. You started out great, but did not hold your boundary and BM beat you and you ended up apologizing. See how messed up that is??? Never be aggressive. There's a difference between being aggressive, passive aggressive and being assertive. Study the difference. It will help you in all of your life. 

Your DH is at fault in this, you know this. I would text one last time in the group chat: "Please remove me from your group text as this is you and DH's responsibility. I will convey my availability, if any, to watch the kids to DH going-forward." And never interact with her again. That's your new boundary. She does not behave well enough for you to interact with, therefore, stop doing it. She does not deserve your attention or help. She is manipulative, a user and aggressive. Low class. Be the real, assertive you going-forward with BM and your DH too, as he needs a lot of work to have your back.