Forest is, in this case, 100% correct. Ariel is making a big show of separating your things; it's reasonable to want to do it properly and get all your things back. And honestly, it's weird and kind of creepy of Ariel to still be wearing your clothes, especially when she's made such an effort to separate your things from hers. Clothes are little cheap things like ballpoint pens or something! Telling her, "I'm no longer okay with you borrowing my clothes, please give them back" is reasonable. If she goes on some sort of "Well you haven't given MY X, Y, and Z back" tantrum, you can respond with something like, "Okay, let's make lists of what we lent each other so we can make sure to give everything back." I suspect Ariel is looking to insinuate herself between you and Dove now. I wouldn't be surprised if she's mad and wants to try to 'take' her from you.