Hi,
I think your daughter and her sibling could make one another happy, as long as one of them isn't the favourite. I've known a couple of people with heart defects or cystic fibrosis who had brothers and sisters, and they were not jealous of their siblings' good health, nor were the siblings jealous of them. It's partly down to how well their parents handled it though.
I think your wife might feel 'guilty' that her daughter has a heart defect. Maybe she fears the second child might have a defect too. Has she considered giving up the unborn child for adoption? Once your daughter is better, I think you'd be in a better position to have a second (in fact, third) child together, without the shadow of an aborted foetus. The only situation where I would consider abortion is when the baby was started by a rape.