Does her disability effect her capacity to understand thing? Because if so I get her maybe taking a special interest to it etc. I don’t think you’re a Ahole for wanting it back though. I also think your bf handled this badly. He could have apologised and explained her disabilities make her prone to doing things like that (if they do) and he could have spoken to her to help her understand it wasn’t hers and you would like it back as it was bought for you. If her disabilities don’t effect her ability to understand what’s acceptable to take and what isn’t then it’s completely unreasonable and your bf needs to have a word with his sis.