Stepson mouthing off snide remarks within earshot behind the scenes playing dumb!?

Unless you want to air your dirty linen publicly on Dr. Phil; you should demand a family mtg. This is abuse and like all abuse starts of with a nudge and then escalates. This person is going to eventually affect your marriage. Sit down and talk; make sure that you are respectful. I know confrontation is hard. Waiting could lead to an explosive one. Maybe this young man has other issues and as a non blood relative; you are an easy target. You don't want him as a paradigm for the younger ones or to change the focus to the younger children. Talk with your wife to assure that she is not afraid or that there has been behavior in the past that necessitated outside intervention-like the police. She needs to get on board. If the family discussions don't work; seek professional assistance. You can begin alone and then see if the family would like to participate. Let a cooler head prevail but again get help. Good luck and God Bless.