I am adopted. How do i tell my parents im pregnant with their biological sons baby?

I was adopted a year ago (I'm 15). I've been going from foster home to foster home ever since i can remember and i finally found one that was willing to keep a girl as old as me. and i was rly glad bc their son, whos 17 was pretty good looking. weve been fooling around for a while now and recently i found out that im pregnant. how do i tell my parents? theyre going to kill both of us, and this is going to ruin the only family i ever had......