I never make a big to-do about coming out to extended family; I rarely see any of them as it is.
I show up to family events wearing pretty androgynous clothing and sometimes nail polish, and if someone asks if I'm with someone, I casually say I have a boyfriend. I'm past being embarrassed or shy about any of that. I don't advertize it, and if no one asks, no one will know. My family is good about not being too nosy, and I'm good about not being obnoxious.
To OP: I would suggest you don't make a big announcement, really. The best way to come out about -anything- in my experience is to be so nonchalant that the person being told usually just accepts it as a mundane fact, or else is not given proper time to be shocked. Don't make a big deal out of it, and usually neither will they.
Anyway, bedtime for me!