Ursula, I don't read this as the MIL interfering in her son's marriage. She's not commenting on the way the woman treats her son but how the DIL treats her. I see that the issue is between her and her DIL and the DIL's  perceived lack of  thankfulness. I'm not an MIL but am a DIL and I do know when I give something to an adult an acknowledgement that it pleased the recipient is nice.  However, having said that, I have a son and if it were my son's girlfriend I'd just keep my mouth shut. I do wonder though how the gifts the DIL gives, if any, to the MIL are received. Does she thank her or only the son? A friend of mine has in-laws who only ever thank their son for the gifts she suggests, picks up and wraps. There are always two sides, right?