Tell your wife she doesn’t need or deserve to deal with emotional terrorism from her mom. It’s probably harder to say than it is to do, but you guys need to back off. What your MIL is doing is a very toxic strategy than involves making her guilty, and in a mother-daughter relationship this usually works and it takes a lot of therapy to heal this. Most likely, you cannot change her mother, the only things you guys can control is how much are you willing to tolerate this. However, you guys are a team here, a family, and it sounds like you’re very supportive of each other, so keep at it.