Help soo

My brother and his wife split up and is getting a divorce my 4 year old niece lives with us and my brother (her dad) he’s going thru a lot but anyways we were raised as Christians and him/we are raising my niece the same way but there’s some difference cuz we never did Pokémon and magic and all that bad stuff I’m trying to raise her like that as well buuut he got her a pikachu animal and she loves it and I told her y we should NOT do Pokémon and all but it’s like every time she gets it it’s like she’s rubbing it in my face she’s always playing with it and saying it’s name how can I brake this without my brother thinking I’m undermining him he’s hardly here but she does tell everything and also my mom never let us do this but she’s not saying anything or backing me up with my niece I have basically raised her and ever since my ex sister in-law basically kidnapped her during Christmas she’s been different I don’t want her to think that if she goes on about it that I will change my mind but it infuriates me that it’s like she rubs it in my face idk what to do