They most likely bought the home together when they were married and did not settle their community property after the divorce. Therefore the home would belong to both parties equally. If your mother's husband left a will and left all to your mother, then your mother would owne it equally with the first wife. She could offer to buy the first wife out. She should go to the tax assessor and find out what deed they show and look at that deed where property was acquired to make sure of ownership. If the first wife never donated it or did community property settlement then home is owned 1/2 each.
Every situation is different and you should consult your own attorney to go over all the particular facts in your case. The answer given is only intended to provide general guidance regarding rights and responsibilities.