Seems like your MIL is jealous of your confidence that’s why she has chosan the worst way to let you down. Criticizing on one’s Shakal is ridiculous and don’t you think it’s the sign of low mentality? do you think that she is possessing healthy thinking?
I know that’s difficult but I think you should not let her feel that her ridiculous remarks are shaking your confidence; just be confident and give thanks to Allah that at least you don’t have any mental sickness like the other people have around you.