Would they treat the new baby any different or your children any different if there was a divorce and a new wife? Half siblings occur all the time from many different circumstances. Would they treat a foster or adopted child any differently? Would they treat the baby or half siblings different if any of the children had happened from an affair?
No child should be treated poorly due to any circumstance. It might just be that if they will make a difference between the children it's best they not be around the children.