Yikes, this is probably the reason why my mom NEVER let me spend the night at another person's house, EVER! (And before you judge, I turned out normal, I swear!

) Whenever I asked her if I could spend the night at a friend's house, she would always say no and suggest that we have the sleepover at my house, if possible.
Maybe my mom's strict rules have really taken root in me, but I agree with the posters who said that they would not let their child attend a co-ed slumber party. While it's true that kids will find ways and means to fool around if they want to, I don't see why parents should
provide them with opportunities for that kind of behavior. Even if they're all just friends, I think most (if not all) of us can attest to the fact that at that age, rampaging hormones can blur the lines a bit. I'm not saying that teenagers think about making out and sex 24/7 (I certainly didn't!), but when parents put their children in a situation where there are members of the opposite sex and beds/sofas nearby, you're taking the chance that something might happen.
Of course if you, as a parent, feel like you trust your child enough to allow him/her to participate in a co-ed sleepover, then more power to you! That kind of trust in great to see in a parent-child relationship. But if you ask me, I'd rather not take the chance.
ETA: As far as the sibling issue goes, I don't think I would allow my son and daughter (if I had both) to have sleepovers at the same time. That would just be more kids that I'd like to handle!