Sister troubles: desperate need of help (Indian family)

For the past year my older sister has been drifting apart from me. We used to talk and hang out quite a lot and many people couldn't believe we were related, since we were so close. Ever since she entered university in September of 2014 (she is in her second year now) she became so distant. She never picked up her phone when she went out, she started coming home really late, she stopped talking to me and sharing things about her day (please keep in mind that I am from a brown/Indian family so our values are different) and it began to cause suspicions to arise at home. All she did was lock herself up in her room and shut everyone out, except for a particular person whom she constantly talked to on skype and test messages. Just this year, around January or so, my parents and I found out that she has been seeing a boy of a different culture, religion and race (again, keep in mind that we are a brown family). It became even more hard to talk to her about their relationship and anything else she was involved in outside of home. She always throws tantrums and threatens to run away and say other things when we approach her or ask about this boy and she continues to get mad if we mention him or where she has been for the day. Many other people I have asked for advice have said that she is an adult now so we don't have control over her life (she uses this dialogue as well), but how can she take care of herself when she is not responsible anymore? She's always out late at night, she doesn't tell anyone where she is going and she lies about who she is with, she cannot cook or clean at all, she overspends on things she doesn't need, and she expects us to believe that she is a "responsible adult". I also feel as though her "boyfriend" is a bad influence on her. From what I have seen he seems like a lazy person who cannot do anything without my sister's help. I am so upset because there are so many fights at home now. My parents seem so stressed, but my little sister is unaffected thankfully (but only because she doesn't know whats going on). I want to help my sister but it seems impossible to talk to her. My family is falling apart and everything is on my shoulders, at times it feels as though it is all my fault since I was and am unable to understand and communicate with her. I don't want her to be with this boy. I don't want her to end up ruining her life because she thinks this is love. She's going to end up living an even harder life than my parents if she stays with him. I am so worried and scared, please help.

am sorry.bt first try to be her friend pretend that you are supporting her then lure her into telling you everythng abt ths boy then after that i wl advice further on what to do