I'm bisexual (I identify more typically as pansexual or simply queer) and 22 and have no immediate plans of coming out to my family.
I'm quite fem, but if you're paying attention at all my preferences should be quite obvious. I have a friendly but strained relationship with my parents when it comes to my love life. They have never met any of the men or women I've dated. Both of them are highly conservative, religious, and openly judgmental of anything verging from the heteronormative perspective. That said, I think they know I'm queer and we've settled into a "don't ask, don't tell" situation.
In my day to day life I'm "out," and avoiding the subject with family isn't ideal. If I could see a way to keep them in my life and have them accept me then I would take that action in a heartbeat and it would be wonderful. As it is, I can live with things as they are now.
Unless I find myself in a committed, long-term relationship with someone who's not biologically male I don't see why it's necessary for them to know. It's not like I'm asking my sisters about their sex lives.
If you really feel like you're keeping a horrible secret, then make sure you have a strong support system of friends before taking the step to address the situation with your family. If you're wondering if you should tell them simply because maybe they should know...I'd put the matter aside for now.