My sister is driving me insane? help?!

So my sister went to take a term abroad in barcelona for college. She has always been a selfish and untrustworthy person. But I liked her sometimes despite her flaws. So I am a junior in high school and I am a guy. She came home from barcelona after like 2 weeks of being out there when she was supposed to go for like 4 months. Of course she messed up, as always. Apparently something happened to her, but I can't know. I am just a normal kid, good status at school, simply enjoying life. She comes home and is now no fun anymore, she mops around, crying daily. It almost makes me, a worry free kid, depressed also. She does nothing, and constantly expects sympathy from me and I am reluctant to give her it because of all the things she has done in the past(I wont get into that now). But she ruins my day with her bad attitude and lack of motivation. I simply can't stand her negativity, and her constant yelling at me when I do NOTHING to her, trust me. She flips for no reason, I am not even mean to her. I am done with it...her depression shouldn't have to affect my good life. Any tips to help?