What do you do about a naughty child that isn’t yours?!

🌕 E.T 🌕

My cousins daughter (4) is purposely naughty and literally nobody does anything to stop it. I live with my nan and she’s always over at our house. She keeps breaking my things, my kittens toys, keeps spraying my cats with water, swearing at me etc. Today she poured cold water on my kitten and it’s quite cold with no heating on so I told her she can’t do that and she replies with ‘I can do whatever I want’ so I turned her TV show off and told her she can’t watch it if she’s being naughty. She then cries and screams until my nan comes in the room, and I get called ‘evil’ for doing this!! I’m 17 and have no idea how you’re meant to deal with kids. Any tips?