You are not being unreasonable, his kids are not your responsibility and being his partner doesn't make it so. What would he do if he didn't have you to watch them? His copout line about partners helping each other out is just him manipulating you. You helping him out would be you watching his kids every now and then, not all of the time.

I've been a step mother for 27 years, since my SD was 5 years old. My husband asked me to watch her ONE time, for 4 hours, because he had to work for half a day on a Saturday.

He needs to find other arrangements, it's not fair to you.

Look, the whole point of visitation is for his kids to spend time with their dad NOT to give their mother a break. If he isn't there then they should be with their mother.

If you look around here you'll find others who have been in your situation (treated like a live-in nanny) and the kids ended up-hating them because they didn't visit to spend time with their dad's SI, they wanted to spend time with HIM 

So was everything resolved from your previous post? Obviously you did not cut and run...what happened there?

There's help and there's "help." Helping is stepping in in a bind, going above and beyond because he needs it. BUT - you are not "helping." You are completely responsible for the majority of childcare (assuming the kids are in school, you likely see them more than their dad does.)

I too became bitter for similar reasons. My SO and his brothers needed so much "help." At first i jumped in because i was going to be the best girlfriend ever. But then i realized that these guys were looking for someone to take responsibility for the basic parenting of these kids for a large chunk of time and i noped out. Easier for me because i don't live with SO.

For you, you should probably explain that you aren't able to take on these major parenting duties due to your work and parenting responsibilities. He could get a babysitter, maybe a family member. Idk how late he is out but there may be aftercare programs they could go to after school. Or he could adjust his CO to where he has them when he can be present. More time in summer maybe, if he's a high school teacher and every other weekend during his busy season? There's no point in him having them if he's not there. 

6 days a week?? That is crazy. I presume he gets paid for this? 

These children need to be going with their dad and sitting or playing on the sidelines. They can do their homework or read or play right there. If you have to watch them when you work then he can do it too. 

I would be upset if my husband was out 6 nights a week in addition to his full time job. And that is before being the babysitter. He needs a major reassessment of his priorities. 

What did he do before you were there? How would he manage if you weren't there?

IMO, he has no business alternatingly weeks like that seeing as he doesn't consider his children to be worth his time. 

Does he ever consider the damage he's doing to his children's self-esteem (daddy doesn't want to see us)? Why doesn't he love them enough to actually spend time with them?

"its a partnership and we help each oher out" 

What utter guff. You do not have a partnership because there is no give and take. Well, there is but you do all the giving and he does all the taking. It's time to spell things oiut for him. He isn't the only one entitled to do what he pleases. As from (date), you are going to start going to (activity), three nights a week. He is free to continue his coaching for three nights too but he's expected to take over all the rest at home on your nights off, including taking your daughter to her sports. Like he says, partners help each other out.

If he is an amazing guy... he will be open to hearing what you have to say.  At 8 and 11.. there could be some relief potentially in a year or two...but you also have said his younger kid has some issues that might make his older sibling watching him a bit tough.

What complicates this a bit further is that this is his job.. it's not that he is just volunteering or engaging in his own sports club... this is part of his job.

BUT.. as has been pointed out.  What if you were not in the picture.  What would he be doing? who would watch his kids while he worked?  Would he have to change roles in his school?  get daycare? 

and.. back to he is a wonderful guy... he should be open to hearing that the current arrangement is not working for you.

I might approach it like this.

BF... I know your coaching schedule takes up a lot of time during the school year, but I think we need to look at options for the kids because I am honestly getting spread to thin.  You know I really like your kids and don't mind helping out occasionaly, but it has gotten to be too much between my own work obligations, my own kid's schedules and the fact that I would really like to just have some time to myself if I don't have those things pressing on me.  Again, I don't mind helping in a pinch.. but I can't be the go to solution for your kid's after school daycare and transportation.  I know you probably don't mean to, but I do feel that my role here is taken for granted.. you don't even try go make it easier by doing things like making sure your kids have a meal ready.. so that gets put on my plate too.  If I were not here.. what would you be doing?  Who would be doing the babysitting?  would you just take them along to your games and practices?  Is that an option now?  I can be willing to help out X day(s) a week but for more than that, you need to see what other options you have.  Again, I like your kids.. this is not a problem because there is a problem with them.. it's just an additional responsibility on my that has gotten to be too limiting and is really burning me out.  

Unless this coaching job is bringing in so much income that you can hire a cleaning service for your home.  And Uber eats for dinner.   Why can't he take his kids with him.  Sports are family things. LOL   Think you must really look up amazing guy.  And see if SO picture is there.  
 Good parents who have visitation with there kids don't coach 6 night a week. Kids should stay with BM. but she most likely don't want them  so your the one getting stuck when everyone else is doing what they want 

OP, why and when did he start coaching? It seems odd that he'd coach for perfect strangers but not for things his own children participate in.

I don't think you're being unreasonable at all and not even because they're step-children. Even if all 4 kids were yours and your SO's biological children together, it shouldn't fall all on you. This was one of the main reasons I left my first husband. My ex disengaged from the family and left everything to me. I was a married single mother. Your SO needs to step up. 

 I would laugh in his face. HAHAHA 

NO, I will not watch your kids. Period

You are saving this guy tons of money on free babysitting and child support. They have a decent Mother. They should be with her. What a joke. 

They are supposed to ask what they would do for childcare, cleaning, rides, etc if you weren't in the picture and do that.  An occasional favor is one thing but you are being taken for granted.

A man's perspective.  

It is not okay that your DH takes advantage of you using you as his defacto  nanny.  
 

Tell him no.  The older one is nearly old enough to baby sit the younger.

Define your availability and have your DH manage his child care accordingly.

Occassionally is one thing.  All the time is another thing entirely and should not be happening.

Take care of you.