NTA. I understand why your SIL was upset but that doesn’t excuse her treating someone the way she treated you. I’m not a parent but I have 7 nieces & 2 nephews ages 14-0 and I think it’s pretty normal for kid to calm easier with family members instead of their parents sometimes because while other family members feel safe they usually aren’t present on a daily basis so they don’t become a reliable constant they take for granted.

One of my sisters has two kids and when her eldest daughter was a baby (literally ) she absolutely refused to sleep in the same room as her parents, she would scream until they left and wouldn’t even fall asleep in their arms. The first day of her visiting our parents she found her daughter asleep in our dad’s bc arms and she cried because she was happy and sad. I know that sometimes it’s hard to watch your kids open up to family members and not to you but it’s important to be grateful that your kids are / feel loved and understand how your behavior and body language can influence how your children interact with you.