Help!!! MIL Keep pulling newborn arms!

My MIL and I come from different cultures. She is from west Africa and I am American.... Anyways, my MIL keeps pulling my newborn by her arms!!

For example:

The first time I saw her do it, she was moving my baby up in a better position in her rock n play, but instead of placing her hands under the babies armpits (like normal people) she pulled her arms to move her up. I was honestly in shock and she did it as she was leaving so I didn’t really have a chance to say anything. It literally upset me so much that I almost puked.... I told my fiancé right when she left that she CANNOT grab a baby like that. I explained how babies joints and muscles can’t be strained and pulled like that. A few weeks went by and we never had an issue.

I was recently talking to his uncles wife (who is American and married into the culture also) And she said his mom did the same thing to her daughter but she made it very clear that she can’t do that. That means his mom has been told this before and she still continues to do this!

Then just a few days ago, she did it again! I told my fiancé again so he could translate but his mom just said “they’re African exercises” and “babies aren’t as fragile as people think”

I can’t stand people who don’t respect other parent’s wishes. I seriously don’t know what to do anymore. 😭😭