Im very very sorry for your loss, but personally, I think you did the right thing. Even if the child had a chance to live, and lets say you raised it, I think there would always be a slight hate for it somewhere in the back of your mind because of the way it was concieved. Also, when the child would grow up and ask, "whose my dad" or something of that nature, what would you tell it? Couldnt its life be completely messed up because of this? What would happen later on, (if your single), and you were trying to find the right guy to marry? He would probably find out about this child, and be kind of turned off from a real, lifelong relationship?
I think you did the right thing, in the long run. Im not all for abortion or anything, but for the situation, I think it was right. Now being male, alot of ladies might not agree with me, but thats just my opinion. Sorry.
Also, I hope your sisters boyfriend spends the rest of his sorry miserable life in jail, gets raped himself, and then burns in hell

(sorry if that was out of line).