Sounds to me, that she has more than just Bipolar disorder, it is possible for a person to have more than just one mental illness.
Maybe the person that told you she has Bipolar disorder just used that expression? And if her family or your cousin's spouse knows this has been confirmed by a psychiatrist, do they know if she is she on meds?
If so either she''s on the wrong ones,too much or too little, or just doesn't want to take them.
This is 2007, don't they realize there are treatments for many disorders?
I'm sorry to hear you are dealing with this, and hope you don't fall into the trap of stereo typing those with Bipolar disorder just from this experience.
There are different degrees of Bipolar and behaviours.
"One size doesn't fit all".
I pray that something can be done for her and her family, that will make life for those around them, easier to deal with, especially for you.
Take care now,
DE