My husband and I have a twin (Jack, and Clara). They made a new friend, Josh. He came to our house multiple times and slept over once. They wanted to go to his place. We've met his parents. The looked very responsible. After a couple of visits, my son told me that Josh's older sister (18-19) is weird. When she watched them play a video game or something, she always seemed to linger and weirdly watch them. She attempts to talk to them but doesn't really say anything. She tried to get closer to them but they don't like it when she does that.
It wasn't surprising to me.
When we went to visit once, we talked about our kids and theirs including Josh's sister. If she had friends she wouldn't have tried to befriend younger kids.
One day, our kids were having a sleepover at Josh's, in addition to another friend (Sara). We dropped them off, said hi to the parents, and left. Clara is a very light sleeper, if anyone walks into the room she'll wake up. Since Josh sometimes snores, his parents prepared the guest room for Clara to sleep in (the guest room is beside Josh's, his sister and the parents' rooms).
So, during that sleepover, Clara woke up and saw that Josh's sister was in her room standing by the door. The girl said that the air conditioner didn't work in her room and she felt hot. She apologized and left. I don't know how the girl managed to walk in, close the door, and wander the room without Clara waking up.
After the girl left Clara stayed awake for a couple of a while, then went to check on her brother and to go to the bathroom. She walked and saw the girl leaving Josh's room, where Josh, Jack, and Sara were sleeping. The girl said she forgot her charger in there (she was holding a charger) so Clara didn't think she was lying, but it was scary at night. She told Clara to go and sleep unless she didn't like the guest's room she could sleep in her parents' room, or hers.
Clara said "Thanks, but I'm comfortable in the guest's". According to Clara, the girl was very calm and composed, not fazed at all by her being seen acting like this. As if she wasn't "caught".
As I said, we knew the girl was weird. My husband and I didn't see anything so alarming. But my sister did. I always listen to my sister because I am not good at decision-making and judging, I've always tried to learn from her but she is more socially aware. My husband is just like me. We decided to not let our kids go there unless the big sister wasn't in the house. So, when they told us they wanted to go the next weekend, we didn't allow it. Our kids didn't complain much, but Josh and his parents weren't happy when we told them our kids would only come if their daughter wasn't there because she is questionable and unsettling to be around. The parents were mad and my husband said I was rude.