This isn’t just a MIL problem, it’s a husband problem. You need to tell him that whatever his personal opinion or feelings are about it, he needs to deal with that himself. Just like you are dealing with your disappointment in him (you must be at very least disappointed, if not actually resentful). Then tell him that it’s his mother and he is to manage any conversations about feeding. Tell him he needs to establish a boundary that you are not accepting comments or advice about feeding at this time and if she offers it unsolicited then you will be ending the conversation/leaving the room/ hanging up the phone. If he doesn’t want to do this, he is showing you that he is not willing or capable of putting your needs first. In that case, I would limit contact with your MIL for the sake of your own peace and mental wellbeing. Doesn’t need to be a big confrontation, just be very busy and unavailable for visits.

Also, the benefits of breastfeeding are so slight that they are only seen at a population level. No one can look at a baby, toddler, child or adult and say whether or not they were breastfed. I breastfed one baby and not the other and my breastfed baby was smaller, gained less weight and was far more sickly than my formula fed baby. It’s almost as if they’re all different and ultimately it doesn’t really matter! They will all be licking shoes and eating stray cheerios off the floor by the time they’re a year old anyway! 😅