I don't think my child is any safer at a relatives than with a school friend. Child sexual abuse is often perpetrated by people trusted by the parents, or even one of the parents themselves, so I'm not going to spend my time by not trusting anyone at all. I just use my common sense and hope that it works out safely.
I would currently let DD spend the night at one friend's house. I've met the mother numerous times now, DD was picked up from school for an afternoon with her friend, and I did the same, we've been to the movies together (both mothers and daughters) and have chatted at school and at birthday parties. I would trust her. She's a single working mother, and I don't believe DD would be in harm's way at her house overnight.
I'm wary of asking though, because of threads like these over the years. Some people tend to respond as if you must be a child molester to even invite a child over for the night, and it just makes me nervous about offering. DD would love a sleepover, I'm just not sure how to offer without her thinking I'm some weirdo freak like some forum members have made me feel I'd come across as.
DD is 7 and has spent several nights at my mother's house too. I don't consider that sleeping over, I consider that me scoring a child-free night. lol.