I am so so pissed.

So I (22F) am very close with my sister(30F) and my niece (5F). For context, my whole life our mom always takes my sister’s side no matter what. Anyways my sister always caves into whatever my niece wants. She’s 5 and still not fully potty trained, my sister and Mom swear up and down she’s potty trained, but she still has to poop in a diaper anytime she has to poop. She has to be carried everywhere, my sister tells her to walk but she whines and my sister picks her back up, you get the point. Anytime I say anything about that my sister and mom get pissy saying my sister has so much on her plate but she literally has 1 child and works a regular 8 hour job. And my sister blames my nieces behavior/getting her way because her parents got divorced. However, that’s not an excuse to have no backbone with your child. I love my niece dearly and always will but she is getting to be such a spoiled brat and it’s not all her fault. My sister blew up at me yesterday because my niece was hanging on me and I told her to stop and my sister said “stop being mean to her” when in reality I’m setting some pretty generic boundaries. Anyways I’m not talking to her and my mom is mad at me now saying “parenting is hard and you have no clue”. I’m so pissed.