Trust your instincts, if the parents give you the creeps, it's better to be safe than sorry.
You are so right Tigg...
Even if the parents dont' give me the creeps... what if they have other creepy family members i have never met that just happen to stop by?
Simple, ask who all will be there. If they aren't willing to let you know who will be there, no sleep over. If it's someone you know well enough to know that they wouldn't allow creepy people around their own kids (and yours), then not much need to worry about that, but if you don't know them well enough to know who they would, or wouldn't allow around their own kids, then ask who will be there, or if there's a chance of anyone you don't know showing up. And, tell M to call home if anyone she doesn't know shows up.
When I was a kid, my parents never did much checking up on where I was staying for sleepovers. They'd meet the other parents for a few minutes, but that was about it. I don't think I was ever not allowed to go to a sleepover.
However, none of my friends were ever allowed to spend the night at my house. Their parents always found reasons for the sleepover to be at their houses rather than mine. My parents creeped out the parents of all of my friends. My friends had smart parents and made wise choices - my parents were best left untrusted. I never had any problems on sleepovers at friends houses despite the lack of checking that was done.