Oftentimes, it's best not to be too kamikaze about coming out when you have a pretty good idea your family is NOT going to be very receptive.
For some folks in that sitch, they've found it best to wait until there was an actual, tangible same-sex relationship they were going to pursue or entering into. For others, waiting until they'd left home and had a bit more distance feels more comfortable. And for others still, just spitting it out and dealing with it works best. You're going to be the best person to feel out what's likely to be best for you.
But if you KNOW it's going to be very problematic, it's likely a Very Good Idea to make sure that the benefits outweigh the negatives. In other words, what does coming out to your family net you right now in terms of the good stuff? Is it going to be worth what will likely be a sizable period of stress or conflict in your home?
It's also worth mentioning that none of us can ever control how another person feels. And many people do have mixed or unclear feelings about bisexuality or homosexuality (and it's actually a lot easier for many to accept homosexuality than bisexuality, oddly enough). So it may well be that for whatever reason, your mother may feel dissapointed or confused or angry or sad -- and you need to be in a space her you can allow her that and handle it as compassionately as possible when you do come out.
Editor and Founder, Scarleteen
My epitaph should read: "She worked herself into this ground."
-- Kay Bailey Hutchinson