To me, if there is no couple time, there is no "us." Yes, there are other responsibilities, but there has to be a relationship as the foundation. If not, you are just the help. Insist that he make time for you. Jesus, 4 weekends a year was too much! I think you can do better. 

It sounds like your fiance doesn't have enough time for a relationship since all his time is divided between his son and his job. There are tons of other ways to make this schedule work (like every other weekend with extra days/weeks during summer and breaks). Your FH could also look at getting a job with different hours so that he actually gets to see his son on the weekends he is there (and in the evenings when you are home).

Your FH has set up a very unfair system to you. You need to stop participating in this system. Don't be home on weekends. Move out and live elsewhere even if you two stay together. Leave him. Do something for yourself that makes YOU a priority.

Don't make the mistake of thinking you can make your FH put your needs first. A good man would have done that already. You tied yourself to a dud. It's okay, we've all done it. It's time to cut that anchor, though.

Ugh.

This happens so often and it's always frustrating and disturbing.

The kid has visitation so he can see and spend time with his dad...not so that some unrelated woman can be forced into domestic servitude...causing her to resent his presence in the home.

You aren't a free live-in babysitter. You work. You have your own life. The answer to this is easy. Just tell fiancé and BM that whatever they decide is fine as far as where kid is staying...but you're no longer available to watch him on the weekends. They'll have to make other arrangements for his care.

That's it. You're not available. No explanation needed. He isn't your kid and you aren't obligated to babysit.

Ugh.

This happens so often and it's always frustrating and disturbing.

The kid has visitation so he can see and spend time with his dad...not so that some unrelated woman can be forced into domestic servitude...causing her to resent his presence in the home.

You aren't a free live-in babysitter. You work. You have your own life. The answer to this is easy. Just tell fiancé and BM that whatever they decide is fine as far as where kid is staying...but you're no longer available to watch him on the weekends. They'll have to make other arrangements for his care.

That's it. You're not available. No explanation needed. He isn't your kid and you aren't obligated to babysit.

 make them do the parenting. Stop helping out. It really is that easy. They will be mad but FH will get over it if he loves you. If he doesn't value your happiness over childcare for his son, you have picked the wrong man unfortunately. I think these things only go on at all because kind women, like you, let it happen. Stop letting it happen.

Leave him. If your fiancé cared about you he wouldn't volunteer you to be the unpaid nanny without your permission.

Say no, if SO is with you because he loves you he will find a way to compromise with you. If he is with you because you make a great babysitter than walk away that is not a relationship.

BM gets what she wants (weekends free), fiance gets what he wants (works his own schedule and sees his kid after work), you don't get what you want (time with your fiance and free weekends). And whose kid is this? Right, not yours.

There is no way this child is your responsibility. It was kind of you to help out, but now to be expected to take on this burden every weekend is just unreasonable.

You have an equal voice in your relationship wiht your fiance. You are not there to please him. If the arrangement isn't working for you, it isn't working. Period. That's it. Don't ask to make changes, simply say that you are no longer willing to take on the responsibility for so much child care for a child that is not yours. And then make yourself unavailable the next weekend, even if that means going out of town on your own.

Then see what happens. 1) Fiance throws a fit. This tells you that what he wants is more important than what you want and he expects you to fall in line so that HE is comfortable. 2) Fiance makes other arrangements with BM and/or his job. This tells you that he cares about your wants and needs as much as his own. Which do you think is most likely?

No offence intended OP but ... no, you are not being selfish, no you are not being evil, but you ARE being a mug. This kid is not your responsibility. He has two parents and you are not one of them. You do not have to take care of him at all so stop it. Make plans for your weekend and do not be at home when bm drops him off. If yoiur bf wants to see his kid on the weekends, let him go pick him up when he's off from work. Do not do it for him.

Personally, I don't see any point in being in a relationship with someone who never has time for me. Why are you sticking around and being treated like a servant?

I totally get what you are going through.  My SO does the same with me... I work 7-330 M-F and he works 7a-7p 3 random days a week.. So guess who gets to watch the kid on nights and weekend days that he works and has the kid.....it's very frustrating and I'm not even sure how to deal with it, I just wanted to let you know that you are not alone! I hope you figure out how to solve this issue so that YOU can be happier. Good luck!