Good for you Lulu.

TBH, I'd be doing the same and for these reasons:

Underage alcohol I don't mind, drunkenness at any age I do. I don't believe the alcohol will be properly supervised at this party so that's strike one.

Will the boys be sleeping over too? Mixed sleepovers in the teens... yes, I was doing that but TBH I wouldn't let my children unless I knew ALL the kids there. And there were seperate girls and boys rooms. And the parents were nearby. And everyone was sober or at the very least not drunk. In fact, I just wouldn't allow it. I know I did it but now I'm a parent it's different!

The third point is that you don't know the family. If my children were to sleep out I'd make sure I knew the family so I knew there stance on things like mixed sleepovers and alcohol. If I disagreed then no sleeping over. If I didn't know the family or wasn't comfortable asking about what's going on then no sleeping over.

You're right, the parents don't sound that responsible and I'd be worried too. I would never throw a teenage bash with alcohol (a quite meal with wine is one thing, a party with booze is another because even a responsible parent should know you CANNOT supervise this all the time... even as teenagers you can't supervise your friends properly, I know from experience in holding my mate's hair back from the loo at her party!), nor allow loads of unknown kids to sleep over. I'd rather my children and their friends be at my house with supervision and undrunk than me worry too.