AITA for refusing to give my brother the gift I got for my MIL even after he and his wife went into my room and took it?

I 27F have a brother who’s 32. He has two kids(6 and 3) and a wife. My parents wanted us to spend Christmas at their house this year so I got to their house on the 20th to help with some of the cooking. I had gotten a gift for my MIL since she loves the same stuff as me and that’s my gift for her but my brother and his wife saw and were shocked that I would get something so thoughtful for someone who’s not family and not even blood family at that.<br><br>They tried convincing me to give it to them since they could really use a gift that good for their kids but I refused. They tried guilt tripping me into getting it for them by saying we’re family and I should want better for my only nieces and nephew but I refused.<br><br>On the 23rd, I woke up to find that they had gone into my bag while I was sleeping and took the gift but they left a note saying that if I tell our parents, they’ll deny it and our parents know they’d never steal and they know I would never lie so they’ll just think I’m jealous of my brother and his family so I should just keep my mouth shut if I don’t want to be seen as an AH. They left it in all caps. I was just so shocked that I didn’t know what to do. And they’re right. Our parents will believe them over me since I “tend to exaggerate a little too much sometimes”.<br><br>I found the gift wrapped under the tree with my brother and SIL’s names on it. I took it back and put it in my room and they were livid. They tried acting like I was the AH for taking back the gift they ‘bought’ for their kids and I told them I was getting it for my MIL. They told me that they knew I was getting it for them as a Christmas gift and that I was lying when I said I was getting it for my MIL. <br><br>They got so mad at me that they went back home with their kids and didn’t spend Christmas with us. I’m still mad over this and I just want to know if I was wrong here.<br><br><br>EDIT: <br>I’ve gotten quite a few questions about why my brother and SIL thought I got them the gift in the first place so I’ll just say that this has happened before. I’ve gotten them gifts that were just as thoughtful and expensive in the past and they’ve led people to believe it was them who got it for their kids and not me and they didn’t correct it because it made them look like good parents. Which is why their kids know that any gifts they get from me aren’t really from me but from their parents.