AITA for stealing my sisters cat because she wants to declaw him?

I (M24) took my sisters (F22) cat home with me tonight because I overheard her talking to our mum about getting her 1 year old cat an appointment to declaw him! I was outraged but I didn’t say anything, I couldn’t believe what I was hearing. I don’t think she knows it’s illegal, let alone the physical, emotional and psychological harm it does to them. I don’t even own a pet yet but I could never imagine taking them to go and get mutilated because you don’t like your sofas getting scratched? I don’t know what to do, I acted on impulse and she’s going to know in a day or two that he’s “gone missing” I just can’t let her do this to him but we don’t get on and she wouldn’t listen to me, even if I tried to educate her. He’s chilling on my sofa at the moment and he’s so cute (I’m a dog person) I feel guilty for essentially catnapping him but she would be breaking a law for one since we live in the UK, and it’s just downright irresponsible.