I was in a similar situation but, always known to be moody, had depression during my teenage years that was known (not much done about it but, known) and didn't tell my family about the bipolar diagnosis 'til I was about your age.

I hit a really bad depression, had to drop out of school for awhile, you know, the non-functional works and just told my parents what was going on.

Mom tried to argue, doctors don't know best (and they don't!) But when I told her the meds were helping a lot and she spoke with my siblings, the picture came to light and she worried, for awhile. Now? It's as per usual.

Dad encouraged me not to be ashamed of it. It's just an illness like anything else but, affecting my mind. Hearing that actually helped me stop fighting so much.

Now, it is business as per usual but, bipolar tends to run in families and my siblings, one who has SAD, has the benefit of knowing they're a bipolar relative which can help loads come Prozac mania or nieces or nephews start developing issues.

The initial shock was what I feared too but, they are your family and they know you. "Bipolar" won't cloud their judgements that much, will it?