NTA - just to forewarn you from personal experience. My now husband’s sister is like Conner, never told “no” horrific brat of an adult. She is 25 and still lives at home, my MIL pays all her bills because “she’s starting a business” when in reality she’s doing drugs while living rent free in my MIL’s vacation home, which she wont let me MIL visit. In the three and a half years I’ve been with my husband she has ruined every family event she’s attended by throwing a fit.

We chose to not invite her to our wedding

Listen to that. We didn’t feel that we could invite her to her only sibling’s wedding she is such a demon. That’s where Conner is headed. His own family members don’t want him to attend events at four because he’s a fucking menace.

Your sister may not be hitting her kid, but she is crippling him for life with her enabling behavior. What is Connor going to do at his first job? Throw a tantrum if he doesn’t like what his boss says? That’s a pink slip. Second job will be a repeat and so on.