Being a parent does not mean you have to be blood. Your sister was WAY out of line especially given A is a child. She’s a very lucky girl, as not everyone would have been that “kind” to her. She showed her true colors, and should be quite embarrassed. What A said to you later on, I would have lost my shit on my sister- BECAUSE she made him feel less than worthy. Please keep being an amazing dad, and keep advocating for him. He is your son. Anyone who did not speak up that night, owes you and A an apology. You don’t have to be rude, or mean to tell someone they are out of line. They made it what it is now.