This is really not the frum for estate planning and that is what you are asking here. She will always have more rights as hiw lawful wife.  If the assets they hold are jointly held with rights of suvivorship then they will pass to her. If not then she gets nothing legally because she is not his spouse.  Please have a lawyer go and speak with your brother.  If he wants her to collect SS benefits as a spouse then they have tio be married.  If you ar speaking of an insurance policy then he can designate her as the beneficiary on the policy.  Insurance benefits like health insurance etc., would not be in place if they are not married.  Good luck.