Of course it doesn’t have to be a big deal but it’s weird. Regardless if you’re family or friends. It’s more acceptable if they were family because no one would bat an eye since there is no sexual or romantic interest between family. (Well there could be but it’s illegal in most countries. To have any sexual or romantic relationship with someone you’re related to. That would be considered incest.) But in this situation/context if the woman was the babysitter it would be weird because that a stranger. Even if you say “he could be helping her” there is many ways to help her without having to hold her waist. No one would be comfortable with seeing their partner holding the opposite sex or the same sex waist especially if they’re a stranger. Even if it is for a short period of time.
Does it matter who it was?
Why is it ok if it was the sister but not another woman?
Do you guys realize that if you're considering the gesture as inherently romantic or sexual in nature it would be weird if he did it to the sister?
But like it's not, it doesn't have to be
It doesn't really matter if it was the babysitter cause the gesture can just a polite way to like guide her out or something
A man touching a woman's waist doesn't have to be a big deal