am I nuts or him?

I learned that on my childrens two day visit with their Dad that they spent one night with the neighbor. She has expressed way too much interest in my son and calls him nephew which makes me sick. I don't know her but I just feel like its wrong for my sons to sleep over at her place. I just ticked off my ex because I tried to talk to him about it. I asked is she going to die trying to save them? Is she going to be aware if my son sleeps walks? My ex tells me I should invite her to lunch and talk to her about my concerns. Hello?? He only has them 4 days out a month. Is it asking too much that he keeps them? It's hot isn't a good enough excuse. I don't want to turn my ac on yet so I will just send my kids to the neighbor. I don't think so. If it's a serious enough problem then the children could just stay home but being hot isn't it.
I am the Mom and I should be the best judge on what my son should be doing and going. He has never done a sleepover. The closest he has came is sleepaway camp and tomorrow he has a lock in at school. Am I right in fact that these things are quite different than just staying with the neighbor. This is the same neighbor that my ex admitted that Alex would go hang out and fall asleep and not come home. What responsabile person is going to let the teen fall asleep and stay asleep and what responsible dad goes to sleep and not knowing that the kid is in safe??? Sorry for venting.