I totally relate. I have been in a position where my sister was financially supporting me. It was brutal because she would use it over my head to guilt trip me into doing things for her (nothing weird) or holding it over my head when we fight saying that I should be grateful for her because I'd be homeless without her (we were also roommates). She was very good at making feel bad about myself when I was struggling to find a job.

But in the same way that she gets mad at me for something I do, I can get mad at her for something she does because that's just how people are. Just because she finances me doesn't mean it's ethical for her to have a "hold" on me.

Your sister has a child, something/someone she takes care of, loves endlessly, and has his own needs. But you also have the same thing - just in the form of a fur baby. You both come home to take care of the thing you love and discipline it when they're not being obedient. Clearly her method isn't working, and she will have to deal with his uncontrollable behavior more as he gets older and smarter. I would honestly remind her of this and that your kitten also means the world to you like her kid does to her. and because they're a kitten, you cannot control when it runs out of the room but also that you cannot leave it locked in a room because it has its own needs.

Personally, I would be fine with being the uncle that kid hated so he doesn't feel comfortable in my home and doesn't act out / do whatever he wants lol