I'm also bipolar so I might be able to help:
If possible talk to a doctor (preferably a psychiatrist) or counselor about how to approach her best and see if he thinks it might be bipolar disorder because she might have a borderline personality instead or possibly even have both.
Most importantly, unless she is a danger to herself or others she cannot be forced into treatment.
Here are links to a list of resources and a forum you can ask questions. It may take a day or two for your new account to be approved after you register on the forum.
http://www.bipolarworld.net
http://www.bipolarworld.org
When you go to talk to her, talk to your dad for help and create a calm atmosphere to sit down and talk. If she is any little bit agitated it is not likely to go well. Don't just tell her she needs help, have some information available on where to go for help. If your parents have insurance, they will probably have a website that lists behavioral health providers where you can find information about psychiatrists.
In the even that she does not receive the information well: MAINTAIN YOUR COMPOSURE AND DO NOT RAISE YOUR VOICE NO MATTER WHAT HAPPENS!! !
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"Gun control is like trying to reduce drunk driving by making it tougher for sober people to own cars."
- Unknown
"A fear of weapons is a sign of ret*d sexual and emotional maturity."
-Sigmund Freud