Bipolar Disorder - is it hereditary?

I have a girlfriend, who I love to death. I love her enough that I would be happy to marry her one day - barring one thing. Her mom is, in layman's terms, a psycho *****. I'm fairly certain she has bipolar disorder. She drinks enough that I would put money that she is also an alcoholic. This combination makes her mean and sometimes violent. My girlfriend hates her mother for it, and has told me she never plans on being like that. But isn't bipolar disorder hereditary? What's the risk that she may end up the same way? I couldn't stay with her if there's a risk she'd end up like that, but if it's not really something I need to worry about, then I would love to, because I love her with all of my heart.