Read other letters about this article
It seems to me that there are two problem areas: the MIL and the DIL.
Hardly surprising, the MIL carries a lot of swing weight within her family and that swing weight is compounded by the inherent power of being a "helpless" little old lady. Confronting a LOL over anything is hazardous in our society; confronting a LOL MIL over off-handed sleights at a family gathering is pointless and foolish. As Cary pointed out, you're not going to fix MIL and you're never going to dissipate the power a LOL has gathered over a lifetime. It came as a shock to me to learn that a LOL MIL can seem sweet, caring and fragile while swinging an iron fist. The results can vary from personally painful to damaging and destructive to a marriage, all depending on how the target reacts. Personally, it helped me to recognize that frail and helpless LOLs are rarely either and that I can't (and shouldn't) change that. From that recognition and having given up any notion of fixing anything, I find myself more often amused than hurt or angered by her ability to manipulate the kids. And she really is a sweet LOL.
The other problem area is how the DIL deals with the abuse. As Cary well pointed out, this is the area under her control.
From the letter, it seems that DIL takes the MIL issues and makes them her own. If DIL, in turn, makes some of them her husband's, what should have gone out the door with the MIL's departure now becomes something broken within the marriage that burdens both husband and wife. Therein lies real danger. Speaking from first hand experience, the long term effects are corrosive. And, adding insult to injury, the son is probably less qualified to deal with MIL than the DIL; he is, after all, his mother's son and MIL (not being quite as "helpless" as she seems) can and will deflect his concerns without effort.
In the end, I think there are two things the DIL can master. One is to give up being sensitive over the things you can do nothing about (Dr. and Mrs., for example, isn't an issue worth any effort; smile (or smirk) about it and move on). The other, as stated by many letters, is to learn to say NO (no you may not continue to hold the baby, no I'm not going to fix dinner, no you may not join us for Mother's Day). More than that, I don't think you can do.
R