My sister claims that because she is the oldest my mothers house will go to her. In her will my mother leaves the house and contents to my father, with everything else to be shared between us. However my father has died since she made the will, so surely the house will go into mums estate and the proceeds will be shared?
Your mothers will probably contains a clause anticipating the possibility that your father would die first, and stating what will happen to the property in that event. Failing that, the house will, as you say, form part of your mothers estate and be divided equally among the children. The fact that your sister is older than you is irrelevant.