My Family Is Crumbling And I Wish I Knew What To Do.

I am starting student teaching in january and am now living with my family which includes me, my mom, and my younger brother and sister. While we aren't as bad as some family's things are really crappy. First of all my sister is now on academic probation at school. She has had two semesters of getting below average grades and she doesn't seem to want any help. She always complains how she sends stuff to her professors via laptop but it gets lost in cyberspace so she get's incompletes a lot and also she doesn't like to talk to her professors or get help for her classes from tutors or counselors. On top of that, she only really has one friend who isn't exactly the type of person she should be hanging out with. She doesn't really do her homework and isn't really a committed student. I don't know why she has these issues, i remember in high school that she actually started to do okay. She lost a bit of weight (not that she was ever really fat, but she dropped from 195 to 175) and was very involved in speech and drama. Now all she does is read books, watch movies and eat junk food at home, and while she does her homework, she has a crappy attitude about school and life in general. I know part of it is she's only 19 and a sophmore but seriously i don't know what is up? her life is falling apart. She doesn't even really have a goal in life other than to graduate school, and is struggling as an English major in spite of being a decent writer. She also is struggling with weight now and is probably 230 lbs and her teeth, which are naturally bad continue to get worse because she wont give up drinking soda
As for my brother, he is 21 and is a high functioning Autistic. He hasn't really done much since high school other than one summer where he worked at our small town street department. While he does a lot of work on the farm, he doesn't have much else. Basically he has no social life and he looks up Star Wars and Star Trek junk and watches movies and those showsin his free time. I feel for him though because there isn't much he can do on teh job front and my sister always picks on him for little things like singing and watching little kids movies and just plain odd behavior. The picking on him drives me nuts.
As for my mom, she's actually pretty normal. She works as a teacher and really loves her job and enjoys her student's in spite of the stresses with the jobs. At home she is okay too, but with us kids its tough. My sister is mean to her as well as me and my brother. Since me and my sister are at school she is at home with my brother and they don't usually talk about much other than how the day went and while my mom has a lot of colleagues and friends at work, her life centers on our home and i feel like it's killing her, but i don't know what to do now that it looks like two of her kids will be at home being unemployed bums.
As for me, things are better but i still have issues. I am an honor roll student and have a good amount of friends, but stresses at home really worry me. I always worry about my sister and how she will do now that she only has one semester of school left and i don't expect her to graduate and basically become a lowlife. As for my brother, i don't know if he can ever be on his own and since my mom is 61 i worry about what he will do when she is gone . I also worry about my health a lot and am practically a hypochondriac since i worry about having a heart attack or a brain tumor for some reason and get mad when my sister starts yelling and picking on my mom and brother. However my positive is is that i am student teaching currently and will be done in May and hopefully will find a job away from home. However i'm so worried about the rest of my family. I mean we don't have a male role model since my dad died when i was fifteen (i am 22 now) and I feel like were all just lost and that my bro and sis are going to struggle which will only make mom less happy and make me wonder what else i can do