The mother has, as spouse, equal rights to reside in and possession of the marital home as the stepfather. Among other things, that means she can let her daughter live there. The stepfather needs to wait until she passes; then he can instruct the stepdaughter to leave. If she does not, he can file an action for "ejectment" ("ejectment" is eviction for non-tenants; e.g. guests and other people you have allowed to live there without paying rent) to remove her. It would be a good idea to let an attorney help him at that point.