ETA after your SWH:
Absolutely, if you hate cutting hair and have difficulty with your FIL, don't continue. It sounds like you've handled it pretty well so far. If your SIL balks at babysitting, I hope you'll have the courage to bring up the kitchen project.
Original:
You actually remodeled your SIL's kitchen without getting paid? Is she the same one who babysits for you? Am I correct in thinking that you don't pay her to babysit?
It seems to me that she owes you free babysitting after all that work you did in her kitchen. Honestly, you need to tell her that you have come to feel that the family depends on you too much, and that you need to scale back on that. Tell her that the kitchen project was a large one and that you are sure that she would agree that the babysitting is can go towards thanking you for that large project. Hand her back her HEMMED pants and tell her that you are sure that she can find someone else to do her mending. Tell her that you don't have enough time to rest, so you aren't taking on non-paid projects.
As far as your FIL is concerned, if he is a nice man and acts like he loves you, once every other month haircuts are a way for you to connect emotionally with him. Please consider continuing to do that.
Don't do anymore "jobs" for the family. When they call and ask, just tell them that you are too busy and too tired to do free jobs anymore. Say sorry and smile, but keep your resolve. They actually do know better, but they need to be shown. The babysitting issue will be your hardest. But if you say it to your SIL the way I've detailed above, you will have made your point clear and then you will see the MEASURE of your SIL. If she refuses to babysit for you anymore because you don't take on her jobs, then you will know that she is just a user, pure and simple. Maybe it's time for you to know if your family members love your for YOU, or for your free work...