disappointment

If this ever gets read, it is no big deal to me...I am just trying to keep from saying things to the people that I really should be saying these things to, but don't want to have anymore reasons to quit speaking to them.
Disappointment runs high lately. I am disappointed with the person my parents had for my sibling. She can be quite nice, but only to those that aren't biologically related to her. All of my life she has been a thorn in my side. Once in a blue moon, she is nice, but most of the time she is just plain mean. We live together for now because we are both finishing our degrees, but when I graduate I will be gone. I will be done in December and I cannot wait. Considering we are adults, it gets old that she calls our mom and complains that I don't help her study...she is an adult, has no learning difficulties, and has always made good grades, but apparently that doesn't matter to our mom. I actually got the third degree for not offering to help her study. The reason why I don't offer is because I am taking 18 credits hours this semester (just like last semester) and I work 24+ hours every Saturday and Sunday. She isn't even employed right now and isn't taking a full course load, but that makes no difference to our mother. But of course, I am the bad child for not helping. I also have two children and they are very involved in activities during the week. She only has one and her ex-inlaws live 4 miles away, so they help with him. I, on the other hand, moved here away from my family and have no one blood related (or obligated) to assist me. I help take her kid to school one day and she takes mine one day and drops them off for church on Wednesdays at the beginning of the semester. The main reason I am disappointed is because of my mother. She is always telling me to be the better person blah blah blah, but doesn't expect my sister to back off or take responsibility for her own actions. I also got "in trouble" because I started running with a friend of mine between classes. Yep, running, you know on a treadmill or out on the open road. Reason I am a bad person for that is because I didn't offer to run with my sister. Umm, that would be because she doesn't run or exercise in any fashion. I don't know where it is written that I have to have every moment of my life revolve around her, but apparently I missed that entire chapter in the sibling book. Wish I could say things are looking up, but they haven't gotten any better in the past 7 months, so I have decided to complain on here once in awhile just because I can.
I know there has to be a better way, but the only way I see is out of her world.