This Christmas my brother, sister-in-law, niece, mum and dad came to ours for Boxing Day. The plan was to all go for a walk, then I would cook lunch, then we'd play games, etc. When my brother and sister-in-law arrived, my sister-in-law said that the niece was tired, and would I mind if she stayed at home to rest rather than coming out for a walk. I figured that if they were happy with her being home alone (she's 11), I was too.
At the end of the day, after they had left, I found my niece's slippers next to my bed and the duvet ruffled. This means she had most likely gone into my and my husband's bedroom for a nap. The books on my bedside table were also rearranged and the drawer to my husband's bedside table was ajar (we don't have kinds at home so we don't necessarily hide away condoms etc.).
She hadn't done any damage, but I was a bit annoyed because a) she had gone into a private space (the door was shut) and looked through our stuff b) she hadn't asked - if I'd known, I'd have told her to go into the guest room.
Should I just let it be, or should I mention it to my brother and sister-in-law, or to my niece herself?