Can you chat to her on a good day about how she thinks you could help best when she's really low? Has she had the diagnosis long?
I have been through a lot with my sister who's not bi-polar but very similar, in and out of hospital, suicide attempts etc. I went through a phase of almost helping too much, and got worn out myself trying to help solve all of the various issues that got her down. She also began to rely on me rather than learning skills to help herself (like not drinking too much, taking her meds and looking after herself).
We've talked loads about the impact of her illness and have now got a good balance and really good relationship because of it (though this has taken years). She knows if things are at crisis point that I'm here to listen, but also knows I have my own responsibilities too and that I can't be there the whole time. This has made her more resourseful herself.
I found I had to be able to work out whether my sister's thoughts about suicide were a real risk. If I listened to her talk about it really hard without being really shocked or frightened (bluddy hard) she'd usually say something that showed me she wasn't sure about it (like the fact she was talking to me about it, rather than going and doing it). I could then usually get her to agree that she shouldn't do anything unless she was really sure and eventually agree to delay acting on any thoughts, (don't tell her to stop thinking about it, she probably can't) while other help was sorted out (which i made sure I was able to help with - organising appts etc). If you're not comfy talking to her about it though, it's really wise to put her in touch with a professional (GP, HV, samaritans)and make sure they know about the suicidal feelings. If you think she's really serious about suicide, and can't get her to agree to anything then you have to think about emergency help asap.
I hope this hasn't scared you, but honestly,listening to and talking openly about the darkest feelings is a major step in helping someone overcome them. It is a balance between listening when she's ready to talk about the scary bits, and being there for fun, smiles and laughter too. My sister and I now "take turns" at offloading, and we both know when either of us need more help than the other. A big learning curve for you both, good luck.