What Borrego60 said x2. Regarding what to tell the big sister in waiting, that's a tough one. If she asks, go from there I think is what I personally would do letting her know that things didn't work out and her little sibling is in heaven.
Not to bring up something sad, but the last few days I've been thinking about my child that left us too early.. I now have 2 kids and love them dearly, but our child that was never born will continue to live on in my heart.