If he has made a will 1 ) since or at the time of marrying his present wife or 2 ) before marrying her but expressed or plainly intended to take effect on that marriage ( " in contemplation of marriage" ) then that will is valid and whatever it says about the house in which his son lives will dictate what happens to it. . If it passes to his present wife then she may do whatever the owner of that house is allowed to do, including evicting your cousin (assuming that he wasn't a paying tenant or holding some lease of years of his father). The uncle should be invited to change his will to give the son the right to remain, that effect being pointed out. He might not have thought of that as a possibility. He might think that his new wife would never do that. He might mistakenly think that the son has an automatic right to remain.If he is plainly set against the idea then at least the son knows where he'll stand.If he is in favour then he should get a solicitor to prepare the new will or make an addition (a codicil) to the existing one. Don't try to do this as an amateur!