You could buy a copy of the GAPS book and give to them and just say, "It's explained in here."
I think trying to explain things to people can get very difficult, especially if they're not inclined to take advice from you because of who you are (viz. a family-member).
Give them the resource, and they'll either read it (and, let's hope, take it to heart) or they won't. But whatever happens you put the opportunity in their way, and it's better than getting into arguments.