It's difficult for people who don't have mood disorders to understand and be strong for those who do, especially in a romantic relationship. Just support and being there will make a world of difference for her. Try not to get angry with her if she is making no sense because it's the chemistry in her brain that's malfunctioning, not her personality, and not you.
I've been diagnosed with bipolar for 7 years now..and I can tell you that for a woman, a husband who is supportive and accepting of her, regardless of her mood swings or erratic behavior, is priceless. I know its a tall order, but that's my advice. Take care of yourself too though-you need to be strong for you, her and the rest of your family.