My question is... if there are mixed families, for example his and her kids and now ours. What happens to their house. I know that if one spouse dies, the other get everything, but happens when he/she dies... who gets the house if there are half-brothers and sisters... does the one child from both deceased parents get the house are does it get split between all half-brothers and sisters?

Check your local laws for what happens when a person dies intestate. The laws vary from state to state.