Declining playdates...

My brother and his wife met at a drug rehab facility. They have been married and sober (as far as I know) for about 2-3 years. They have a baby together and she has 2 kids from previous relationships - a 7 year old girl and a 10 year old boy...I just moved back into the same state as them recently, and we are not close. We don't see each other often, and while she seems nice, I don't know much more about the wife than I have previously stated. I have a 6 year old daughter, and at recent family get together, our daughters really seemed to have a great time playing together. We don't live too far from one another, and my brother has been continuously asking me about letting my daughter visit to play with his daughter. The insinuation is that I would not be there for these visits. He has been suggesting a sort of back and forth playdate/babysitting scenario.Under different circumstances this would be fine. I wouldn't at all mind his daughter spending time at my house, because the girls are close in age and she seems very sweet. My problem with the situation is letting my daughter go to his house without me there. There is always a little bit of a concern about former addicts, for obvious reasons, but my main concern is actually his wife's son. I have only met the boy a couple times, but I don't think I've ever heard him talk. He does strange things and comes across as very creepy. At family gatherings at other people's homes, he disappears and I don't see him for the entire visit. I have no idea where he goes. One time, I had run out to my car for something, and when I came back in I noticed that he was sitting on a step, 3/4's up the staircase that goes to the homeowners bedrooms, in complete darkness. Just sitting there. He didn't have a phone or tablet, or video game or book. He was just sitting. Staring. It really creeped me out. I have seen him in a few other similar situations at other gatherings. I have a medical background, and have had training/certification in recognizing and reporting child physical and sexual abuse...so I have heard many accounts of how these things happen, perpetrators are almost always relatives or close family friends. While I've never seen the boy do anything sexual, his general demeanor makes me uneasy. I know he must've had a tumultuous early childhood because his mother (maybe also father?) was a heroine addict. So who knows what he's been through or how it's affected him?TL;DR - I don't trust my brother's 10 year old stepson, how do I decline playdates without him catching on to the fact that I don't want my daughter going to his house ever? There are only so many times in a row that I can say that we have other plans...I'm running out of excuses.Thanks in advance for any advice on the issue. via /r/Parenting http://bit.ly/2HWgBkj