Really not sure about this, I wouldn't say I have a strong view re this either way but I am leaning towards not letting her stay.
Basically MIL has said DD's cousin will be sleeping over this week (she looks after him the next day), he is 8, and does DD want to sleep over too. Now, the arrangement usually is DD sleeps in with MIL when she stays over (doesn't sleep over often) as her and FIL have separate bedrooms each with a double bed. I don't have an issue with DD occasionally sleeping over in MIL's bed with her (although she often comes back quite tired, again, not an issue as is only occasional).
I think I may have an issue with DD having to share with MIL and cousin a) due to different gender and maybe not appropriate? just doesn't sit quite right with me and b) the space issue, if she doesn't get much sleep in with just MIL she'll get even less with cousin in as well.
I made sort of non-committal noises about not being sure what we had planned for that particular day, I think she possibly took that as a no but can't be sure she won't ask again and not sure what to say?! I don't want to blurt out that I don't feel comfortable with her sharing a bed with her boy cousin. DD is 6y if that makes any difference.