I want to first say I have Bipolar and also refuse medications. I have been med free for 10 years. However Bipolar is not an excuse to put your family and loved ones through hell. Quite often people self diagnose themselves with Bipolar as a way of justifying their bad attitude. So I have to ask was she diagnosed by someone since you have known her? Or is this just an assumption made by her?
The second issue is the genetics. Yes mental disorders have been known to pass through generations. My father has it and so far it looks like 1 of my 3 children may have a form of it, although they can not officially diagnose a child with it. If her behavior is causing mental pain for you and she refuses to seek some sort of help get out before you have children. While I do not take medication I also do not lash out at my husband. I also seek natural ways to control the issues I have. A relationship reguires both people to work toward a HAPPY future. If she is not pulling her weight in that why waste the rest of your life with that or risk putting children with someone who clearly doesn't care about those in her life? Bipolar should never be a justification for being a bad person. Tell her to get help, if she refuses leave and move on to someone who cares about how YOU feel!