I wonder whether Paranoid D-I-L's mother-in-law could by any chance be from Israel. I lived there for a few years and was amazed by how many people would eat their way through the (marvelous) grocery stores. If that's the case, D-I-L need only explain to M-I-L that she is no longer in Israel and the Israeli way does not apply here. I had to do that with an Israeli friend who tried using the Israeli way of line jumping at the bus stop one day. She only smiled and accepted the reminder graciously.