I am not really a fan of sleepovers simply because due to size of our flat is hard to reciprocate. But if I were you I would let her go, if she wants to and is keen and has had other sleepovers (so she has an idea of what happens at a sleepover and what doesn't happen at a sleepover). I don't really see it as any different to going somewhere for a playdate. My kids are in the care of people I don't know all the time, at daycare, at art class, at friends places where I only know one of the adults, etc. It is a personal decision though and if you don't feel comfortable with it, I don't think any problem with not allowing it, just offering my perspective.