Living my own life

At what point in life do you get to stop living by others' expectations and do what you want to without guilt. I grew up thinking that my family loved me because they had to, and that none of them really liked me. I had a favorite aunt who really used to take my side and fought for me but she passed away when I was in a 6th grade. I remember it like yesterday I was already sad because I came in 2nd in the school spelling bee (nerd much) and then I get out of school and found out she had died.
Ok I digress, but really I feel like I have fought my family my whole life so then I became the person that does stuff to please even though I really didn't want to. I began to feel used like people called on me when they needed stuff but when they went to do something fun they called my sister. So rather than turn people down I sort of started to withdrawmy self and put on this demeanor that made people uncomfortable asking for stuff from me in the first place. The thing is, that is not really me either, but I prefer being perceived that way rather than being a person who is taken advantage of, but somewhere the real person gets lost.
I think that it's amazing that no one (probably myself included?) really knows who I am, I grew with an older sister I always felt that I had to measure up to, she was the natural athlete, I was a gifted student with athletic abilities, but I downplayed my intelligence to become an athlete like her because I felt more people got excited about sports than how many A's I got on my report card or what honor societies I got into. I remember seeing an old journal where I complained about being fat at 137lbs(now I weigh that in one leg) because at the time my sister was probably a size 0.
As you can see again my mind is all over the place, anyways, my mom is probably one of the most giving people I know, she had to help raise her brother and sister and by the age of 18 she was married with two kids. She has literally been taking care of people her whole life except for about a year she went to live in California and we stayed with my grandmother. She has helped raise her nieces and nephews, random strangers, when my grandmother got really sick, refused to put her in a nursing home and so she cut her business down to the bare minimum and took care of my "Bigma" until she took her last breath....ok now I'm crying. Even as we speak she has two teenage foster sons, she truly is amazing but she has missed out on a lot. She has made a lot of sacrifices but she is young enough to still want male company and do things but the path she has chosen to take is not conducive to dating and travelling.
I love her for what she does but she has this expectation that we should be the same way, right now my sister is working 4 days a week from 3:30pm to 2am and she has 2 kids ages 9 and 2, which means Wed-Fri I keep them until 2am and Saturday evenings my mom does. I have to get up to go to work at 5ish am so needless to say dealing with a 2 year old is not fun. My sister has this expectation that her family is supposed to help her just because, although I get help with my son occasionally and got more help when he was younger, I never expected it. I don't mind helping but 3 days a week is a lot, picking them up from 2 different places, making sure they eat, changing diapers, making sure baths are taken, put them to bed not to mention when school is in I have to help my son with his homework and cooking etc. My sister picks my son up and takes him to school everyday that's supposed to be a trade off, but I don't think so. She spends about 15 mins a day with him, I spend about 30 hours a week with hers. My mom says that we should be happy to help with each others kids, helping is one thing this a job, when am I supposed to have a life? My sister is getting married in October and I am ecstatic for her but when am I gonna have time to find someone and if I do find him when am I gonna get to spend time with him? My sister doesn't like staying home with her own kids, she tells people all the time she rather work than be home with them, she has two great kids but they are very needy when it comes to attention, my son is like me happy to spend time alone, he reads books and can amuse himself. My niece and nephew have to constantly be doing something to be seen, the 2 year old (niece) likes to scream at the top of her lungs for no reason. I love her to death but sometimes when she is here I want to be drunk or stoned REALLY BAD!
I feel like if I stop keeping them it will all fall on my mom, everyone will look at me like I'm the bad guy and if I ever need help with my son they are going to look at me sideways. But yet when I asked my mom to take another day, so we can be 2 and 2 at first she agreed then she said it was too much. But I'm supposed to be cool with it.
Should I care? Should I just revised the terms and say I will only do 2 days? I'm sorry for the long post but I am going through a bunch of things right now financially and you guys know about the surgery frustration and this is just adding to the stress to the point that sometimes I think about moving away from Tampa to escape the expectations....Am I crazy or selfish?