I'm so sorry to hear about your sister. I have Type I Bipolar and was committed to hospitals many times before one had to be a "danger to one's self or others".
There are many medicines (and new ones on the way) that can help stabilize someone with Bipolar Disorder. The problem is that someone has to TAKE them in order for them to work. Many people with this disease stop taking their meds - some because of the side effects, some because they don't think they HAVE a problem.
I wish there was an easy answer. One thing you might do is check out ****. They may have a local chapter near you and they are an absolutely wonderful support group for friends and family of people with mental illnesses.
Are you your sister's legal guardian? If I were you I would find out what the resources are available locally for help - medical, governmental, support, etc. especially if you face having to deal with a noncompliant bipolar family member.
There are others here who are actually in your shoes and they will be able to help even more,