Yeah, not sure how you dodge that bullet. Reconciliation with the parents is impossible since they can never be trusted after what they've planned. Either they die or he dies. As for the kids, they might be innocent now, but they'll have lost their parents and they won't be aware of the assassination attempt on their elder half-brother that doesn't have a good relationship with them. They'll become a threat to him eventually -for revenge or gaining power- so it's natural to get rid of them.
We'll just have to see how she handles this. If the parent can die in an "accident" and the siblings can bond and get closer to each other, it might work out.