Hey.. Dont worry i'm sure you'll be fine.. I'm pregnant.. Just found out last week, and the pregnancy is basically through the same way your child was conceived. We were dating a bit, but no strong feelings for each other. Anyways, when I told my child's father he absolutely flipped out.. He's nervous as hell just like you.
As for your family, if they supported your sister, then they should definitely support you. It's their own blood, of course they're gunna accept the baby.
And props to you for stepping up and wanting to be a responisible parent.. I'm not sure how my baby's father is going to act for the next months, and when the baby is born.. But then, he's only 19.. I'm only 15, so I suppose neither of us really kno what to do..