Dude, in all honesty this is extremely not cool from my perspective. It's your sister; your blood. What if you fuck unprotected? There is a 60% chance the baby will have a defect of some sort. Don't even worry about he baby, think of your parents, man. Fair enough if you pounded a girl that isn't family, but how are you going to explain a pregnant sister? You'll get more beatings then you've done to your dick, dude. Trust me, tell her yourself that there is nothing between you two and stop whatever you're doing.