Some good advice here with some angles I hadn't thought of. That's why I posted the question here because I know I'm unlikely to think completely rationally about the situation.
Step one is going to be to dive in feet first into my own eating plan. If I don't do that then the junk food is pretty much a moot point. If a month from now I'm still on track and the junk food is still a major issue for me, I'll address it with everyone.
I talked to my wife about some of my concerns with our guests (not all diet related) and she's as frustrated as I am with what her sister is feeding the kids. She's talked to her about it but the sister is afraid that if she doesn't give them everything their dad does, then they are going to choose him over her. Lot's of different dynamics going on with that situation.