When a parent dies and has two children and the estate is left to only one what does the other child do?

Legally, assuming duress was not used and the parent was of sound mind there is nothing the sibling can do who was cut out of the estate. Unless the disinherited sibling had some major character issues or had already borrowed his/her share of the estate prior; the sibling who has inherited the estate should split it. From personal experience with a spiteful relative I know this is the best course of action because: 1) It's the right thing to do. 2) The inheriting sibling wants a relationship with their sibling.