Hello all, I'm keeping this off my main as my family members know my acct and I don't want them asking me about it just yet. To set the background, I'm (29F), my fiancé is (31M), and I have three children (8F, 7F, 5M). Sometime last year my daughter Summer (the 8F) started collecting Pokémon cards that her bio-dad would gift her during visitation. I haven't kept up with the fandom as I aged but collected coins as a kid and loved the idea of helping my daughter grow her collection. I have also supplemented the collection from lucky Goodwill finds. It is about 5 binders worth of cards, with cards from between 1995 to 2018 generally and some are very rare. My SIL asked my fiancé if we could babysit her two sons Peter (8M) and Paul (7M) while out of town for the weekend as she has to pick up her husband's nephew for something. I was not keen on this for a few reasons, being: 1) Peter has to attend an alternative school for anger issues and outbursts, and though they have become less frequent I wouldn't know how to handle it if something happens. 2) Peter has had issues in the past with stealing money from my fiancé and I when we would leave cash out in our room. His mom would get defensive about it but $10 was never something I'd die on a hill for, so I'd usually drop it. 3) Peter has also tried stealing my daughter's Pokémon cards in the past or my DBZ cards. I took them back but it blemished my relationship with my SIL a tad. Despite all this I hoped the weekend could go smoothly as she needed the help, so me and fiancé agreed. However, last night while I was at work Summer called me and asked if she could 'give her Pokémon cards to Peter'. I could hear Peter whispering in the background. I was very clear in saying NO because I couldn't advise if there would be a fair trade or if he would take the more valuable cards. Later I asked my daughter if she really wanted to give him the cards and she said no. This morning while tidying my daughter's room I saw all the Pokémon binders out. I had a bad feeling, opened them up, and discovered HALF THE COLLECTION missing. The binders were full but now there were empty pages and scattered cards missing. I was pissed. I went to Peter's backpack and found some of my DBZ cards, my other daughter's full rock collection, and a massive plastic bag with a bunch of Pokémon cards dumped in. I took everything back and sorted my daughter's cards from Peter's, though I was just going by the card's print dates as Peter only had cards from 2022 and beyond (so I thought). I put the sorted cards in a lunch box. I told my fiancé at the time this, but Peter and Paul have noticed their card stash is much lighter and started whining in saying that their aunt gifted them some older cards that are missing. I'm sick and tired of them thinking they can just steal whatever they want from our house. AITA for not giving them any cards back, even if some are actually theirs? I'm worried they'll complain to my SIL.