Visitations should be rearranged so that the SS is actually visiting his father. If Dad isn't off on weekends, schedule the visits for the days/times FDH is off. You have every right to be free to do time alone with your own children.

If impossible to reschedule (too much overtime or whatever causing the amount of work), tell FDH he best line-up his sitter because it isn't you. It's one thing to occasionally offer your assistance (example, FDH is only going to be gone a short part of one of the weekend days and you have 'family' plans when he gets home), quite another to think of you as the built in babysitter.

Anytime that you are agreeing to babysit, you should have a decent notice and actually be asked and given the choice of declining the request (and declining without being made to feel guilty or 'selfish'). There's nothing selfish about expecting parents to figure out how they are going to both work out a schedule and arrangements to watch their own children. Nothing to feel guilty on your part over.

I think sometimes issues over this kind of thing arise when the stepparent has been 'ok' with it at first and then part way in becomes to resent it and tries to stop the infringing. Better to be up front straight about what you're willing to do, occasionally do, and/or won't be expected to do.

Yes, focus on your kids and have an honest talk with your fiance, you are not the mother of his child. Even he is a good boy, not yours. He should be with mom, dad or anyone in the family.
You are not Julia Roberts in Stepmom (joke)
In my case, my husband travels and works a lot, so my DH and his ex tried to force me to babysit my SD while he is away.
I said to him that I was not going to do that, first of all because I am not her mother, I have two little babies and cannot deal with extra work and also because the kid has psychological and emotional problems, and she does not talk to me. The kid also hurt herself with a fork in the ear, so I cannot be responsible if anything happens.

Be polite, and diplomatic, but it is clear your SS is not your responsibility. If you do that they will certainly ask for more. Can you imagine? And then you will have full custody!!

Amazing what you said, I feel the same way when my SD is coming: anxiety and I want to run away!

Hope you can solve this situation, wish you the best!

This makes me so sad for you! That is crap. You are not the mother of that child and you are not expected in any way, shape or form to care for that child in the absence of their parents. I had similar issues myself. My partner would always find odd jobs to complete when my SD would come over for her once a fortnight, three night visit. "I'll just be half an hour" he would say, then 5 hours later he would finally turn up. I have no children of my own, so for weeks, months even, on end, I was feeling so 'trapped' and stuck at home with a child that isn't mine. She would be well behaved in my care, but that's not the point. I did something drastic. I changed my work hours. I now work the Saturdays he has her. It was the easiest way out of a situation since I'd already chatted to my partner and said I felt like the babysitter. I also tried to get my partner to see that it was completely unfair to SD to leave her with me constantly. She comes to spend time with dad! Not SM. You need to get your partner to see this too. It becomes almost ritualistic chore type behaviour in the end, where the term 'it's your turn or it's your weekend' is thrown around a lot. If you were agreeing to mind your SS in that time, then fine. But, it should never be forced upon you! I made a conscious decision that if my SD was ever left in my care for an extended period of time without reason or asking, I would drive my SD home to her mothers and explain why she was there. BM is such moo moo ('cow') that's it's highly likely that she would refuse to take her, so I would then just call the police. It seems drastic but my parents taught me they for every action comes a consequence. I honestly feel that drastic measures are the only way to go when talking it out hasn't worked. Use initiative, talk to your partner and if he won't come around, build a plan that leaves you looking innocent enough to prevent him asking if that's what you did. He won't ask, because he knows it's unfair and so does BM! Time to fight back Goodluck!