Hi...I am fairly new here too but from what I have read abut Bipolar it does seem to come around the times of hormonal changes such as puberty, pregnancy, and menopause so what you are thinking can definitely be true.
I would strongly suggest that you get your sister in for a psychiatric evaluation....her history and behavior seem to be strongly indicative of her disorder being triggered in a significant way and she needs intervention ASAP.
This is a wonderful place to get support.....hang in there and I am sure those wiht more knowledge will chime in and tell you from their experience what they think.
Is there anyway that you can convince your sister to go to her doctor with you going along for support??? She is certainly lucky to have a sister who cares so much about her.....that is what may be her saving grace and if you can convince her to follow this through you may help her to get her life back on track. From what I have read here a Bipolar person without meds is like an alcoholic who is drinking or a heroin addict getting his fix. Your sister will need meds to assist her with the chemical imbalance that she may be going through at this time.
Good luck and please keep us posted ~ Goody