"Try asking her out without your mother and grandmother. She probably feels she is playing second fiddle to your family, and rightly so."

But you're just reading in that the MIL is left out of the conversation, aren't you? If the LW is in her 40's, that means that her grandmother is likely in her 90's, so it hardly seems likely that she's dominating much of the conversation. It's obvious that the LW feels neglected by her MIL's refusal to go with them. So is the LW supposed to have two separate mother's day celebrations on one of the most expensive restaurant days of the year, simply because her MIL doesn't feel like going out with the whole family? After all, the MIL's son and grandchildren are there, and that's presumably who she wants to spend time with on mother's day. If she doesn't, she doesn't have to.