Maybe I have a different relationship with my sister than you do, but I'd ask her.
"What's the deal with Bambi and her family? I looked on Facebook, and what I saw of her fiancee set off some alarms. They're family, and I want to have them around, but I also have young kids I need to consider here. I don't really know any of these people, and I'm concerned."
Yup, this is pretty much the outline of the conversation.
Like I said, my big is not oblivious. it's her daughter but she's not blind.
I can call her out on it and have an honest conversation. But if I lead with NFW, it's going go south in a heartbeat.
So I have to start with "So, really looking forward to seeing everyone, but I need to talk to you about some things that concern me."
Also, really like K4's suggestion - I somehow forgot how much fun it is to make fun of quirky relatives. This is like Real World Shelter Island, the kooks are what will make it entertaining. Starting to buy into this now.