Bipolar Sister?

Just started to try and find out about bipolar as it has never really came up as what my sister could have. Looking through all the symptoms I can confirm she definitely has the majority, if not all.

My sister is 20 and still lives at home. Growing up she was violent towards my Mum and dad and sometimes to me. I remember sitting covering my ears while she slammed doors and shouted. She used to refuse to go to school because she hated everyone there, not even when a teacher came up to the house to try and get her to go to school would she go. My Mum then got a doctor to try and help her for someone to talk to. I remember we once went and spoke to this women about everything which was happening which helped for no longer than 2 weeks and she was back to her usual violent way.

Now, she seems to hate me even more than she did as she says things which I do not think anyone can take back and even I think our whole family situation would be fine if i wasnot here because she makes everthing seem my fault. I cannot seeing it being all down to jealousy because we have both had the same growing up, sharing a room probably has not helped. She expresses her hate for me one minute then it could carry on for a day and things would be back to normal or what I consider to be a good mood of hers. More recently I have noticed how I can say something one day and she will take it how it is supposed to be took but then I can say something the next day and she will totally think the worst of it and get really angry at what I said. Its like walking on a broken mirror, not sure if one piece will crack. Before my Mum has tried to get her to talk to a doctor again but she goes and comes back saying that they do not help her. She hates everyone at times but other times can get really hyper and excited especially with a drink or get very angry. Her excitement even scares me.

im scared for her because I know she will never see she has a problem, she will never want to sit and talk to someone about what she might be feeling and im feeling that is the only way that things are going to get sorted. Its really hard to try and explain this all and I feel as if I am making no sense because there is just so much I can mention that has happened. My Mum and dad are tiered of it all they have had to put up with the constant abuse which is horrible and I think my Mum feels as if she has been completely defeated and that my sister can control her because she is that forceful. I really worry that my sister will live continuing to hate everyone one day and love me the next, im tiered of this constant roller-coaster. I really cannot see anyway out of this except that a miracle happens and she either grows out of it but I really cannot see it as its been going on for probably 7 years.

I do not know what help I am looking for here just something...