The house cannot be sold without her consent. They both own all of the house, and therefore he cannot charge her rent for living in her own home !!! Whether she has contributed to any bills or mortgage is immaterial and it is likely that in a divorce situation she will be able to remain in the property while the child is in full time education and he will probably have to rent a flat to live with his new partner and pay the mortgage too. I do not know what amounts are involved (i.e. equity) but I am afraid it sounds as if your brother needs a reality check! Surely the child's welfare is paramount and he wants the best for it ...
Sorry if this is not what you wanted to hear, and others may say something different.