This would trouble me if it’s a totally separate house.
The potential problems as I see it would be the opportunity for them to egg each other on to risky behaviours, or to the extent that one or more becomes very upset. If an adult is present in the house there is an opportunity for early intervention (alerted by commotion/noise/smell whatever). Also the knowledge of the presence of an adult in itself is likely a limiting factor upon behaviour.
In my view, however “sensible” individual (or pairs) of 12 year olds might be, the combination of 5 different personalities and the heady sense of freedom that they would have in such circumstances could lead to bother.
Do I think it’s more likely than not things will be fine? Yes l, on balance, I do. But do I think there is a risk something “bad” might happen? Yes to that too.
If this sleepover was on my property I’d hole up in the annex on my own from late evening with a book. The slightly trickier issue is whether, despite it bothering me, I’d let my own DD go. I’d definitely speak to the parents and find out how much checking in there will be and what the methods of communication will be between the 2 houses.
And my decision would ultimately depend on my conversation with DD. I don’t as a parent necessarily rule out all activities that carry risk. I try to mitigate the risk and then find ways of dealing with them should they arise. It’s a balance between acknowledging the risks of the proposed event v the detriment to not going. And in evaluating that latter, emotional and psychological welfare is very important. If this new friendship is very important and the event is very dear to your DD’s heart I might still allow her to go (despite my misgivings) once I had ascertained the detail of the arrangements in place and made plans with her as to how she will keep in touch and/or summon the householders/you if necessary.