A great gift for a Mother in law who loves poker is a trip to Las Vegas, Atlantic City or a more local casino. If that is not in the cards (or budget), you could get her a new poker set with chips or one of those electronic poker games so she can play anywhere.
Most areas have poker tournaments that are sponsered by local fire conpanys etc. You could buy her an entry into one of these tournaments. She would have alot of fun an you would be helping what ever organization is sponcering the tournament.