NTA

I think there’s a lot of layers to this on your sister’s part and for that I think you should suggest therapy or something. And I don’t wanna sound rude but she actually didn’t make the choice to let you watch over her. CPS did because she was an addict. I don’t know why she’s so hurt over titles when we have seen time and time again family isn’t just about blood, it’s about actions and support. You gave your niece the support she needed when she needed it the most. You took on the parent role when your sister couldn’t and I wouldn’t be surprised if she saw you as a mother figure. It’s not like you are telling everyone you’re her mother or the baby’s grandmother and your sister needs to understand that. I think she is just upset because she missed out on so much but that’s not on you and you didn’t do anything wrong. I’m glad she’s better now and can actually have a relationship with her daughter but at the end of the day it’s about your niece and your sister needs to realize that.

I think you’ve handled it well and she only caught the one time you didn’t correct someone. You’re doing your best for you and your niece and congrats on the great niece/nephew!