I did taxes for an estate in which the brother inherited everything and the sister nothing. The brother had worked in his dad's store, while the sister did not. She also evidently loudly accused her dad of killing her mother because she was comatose and he agreed they could pull the plug. He never forgave her for laying that on him and thought the son should have the store and some more money to fund it with.
She sued, and her brother gave her something to settle it, but she really didn't have a case, since he had the clear intent to cut her out, and he had articulated sane reasons for doing so.
Note that Leona Helmsly cut her children out of the will and gave her billions to her cat. The court ruled that the cat could live quite comfortably for life on a couple of million dollars, and gave the rest to the kids because it was just too weird to cut them out, proving she was just a little crazy when she wrote the will.