So weird.... I just had to deal with this very same issue, only farther down the road. I was diagnosed with BPII in college as well. I decided to tell my family about my diagnosis but keep it really general, as my relationship with my parents is far from healthy and ideal. I eventually moved across the country, which made it even easier to hide symptoms (including self-injury) from them. Last week (ten years and many ups and downs later), I did some hospital time on suicide watch. After being off the radar for six days with no explanation, there was nothing I could do but tell my mom what was happening.
I wish my experience came with some kind of insight or advice. It was basically non-eventful compared to all the other stuff that happened in the psych ward. My mom is in her mixed state of lots of questions and denial (in ten years, you'd think they'd at least do a google search on BP?!), and I'm still glad for the cross-country buffer so i don't actually have to see her. But the reaction wasn't as difficult as I expected, either.
I pray everything works out for you... the bottom line is that you can't do this alone. You need support- whether from family or really close friends.

Sending hugs your way!