I hope your brother is discussing the matter with his lady friend, and not just with you. If this is the way he is thinking she may decide she doesn't wish to get married anyway.
If they do get married, then as his wife she most certainly will receive some if not all of his estate, if he predeceases her. Much depends on whether or not he makes a will, or not, and whether at the time of his death he has any existing legitimate children. As the previous poster said it is complicated. It does make me wonder what happened to the idea that a marraige is an equal partnership.