SIL embarrassed my daughter at a family reunion

We went to a family reunion last weekend and I've been working on potty training my daughter. She's 2 and she's had a few slip ups but she's usually pretty good. The reunion was out in the country with no restrooms except a portable restroom. My SIL doesn't like me because she's a very competitive person and she doesn't like me because I got married first and she wanted to get married first. We were both pregnant at the same time(but I told my MIL that we were trying so my SIL decided to try too) she wanted a girl but had a boy so it pissed her off even more that I had a girl. But at the reunion I had my daughter wear a diaper that day because I didn't want her using the portable restroom because it was absolutely disgusting. My husband told my SILs and MIL that we were working on potty training her. So when they saw her with a diaper on at the reunion they went around telling people that my daughter is getting too big to be wearing diapers(my daughters kinda tall for her age) then they were saying how I'm a bad mom. While I was helping them set up the food, she let my daughter sit on her lap and she got a wet diaper and she felt it so instead of telling either me or my husband she decided to push my daughter off her lap and whoop her for wetting her diaper. I wasn't notifed of the situation until the next day because my husband didn't want to cause a scene between his sister and I. Next time I see her she will be confronted about the situation because nobody should ever treat someone else's child like that.

As much as I want to do this to her for putting her hands on her...

I'm going to remain mature as an adult and confront her about it.

Update:: I confronted her in front of my SO and MIL and she denied everything. Then when my SO got in her ass about it she said how my daughter deserved it for wetting herself. I was thinking, this bitch is out of her mind. I didn't even notice I was getting in her face until my SO and MIL got between us. I told her if she ever laid a hand on my daughter again then she'd lose it. She's not allowed around my daughter period. Cutting off all contact with her. My MIL kicked us out and yelled at me for disrespecting her daughter like that. She disrespected mines and she's 2.