Spanking one year old

I was at my sister in laws house (with her parents) and my one year old (13 months). Her old year old was repeating playing with non toy item after being told “no”. Multiple people were shouting “no” at this one year old (15 months). Eventually her mother picked her up and smacked her bottom. The baby then yelled and cried while her mother excused her “tantrum”. Grandpa told the baby he’s heard her “yell louder” as if her tantrum was fake. I practice gentle parenting and was appalled at this behavior from adults. My husband wasn’t there and I’m not sure how to address it with him. I would never want my child treated that way which makes me not want to ever leave her with my in-laws. My husband thinks i am unfair to his parents already but who encourages this type of “discipline “ then belittles a child afterward? Even if spanking is your thing… isn’t 15 months a little young? Like whatever lesson they think is being learned isn’t. She’s just too young to even understand

Edit: To be clear: we never leave our child in their care