special days cause so much trouble

Ignore her and get on with life with DD and DGD. If she wants to feel jealous, that is her problem, don't be nasty to her but don't rearrange your own lives to suit her either.

soontobe, the answer is no. We cant understand her, she has a good job is in her late 50s and her husband is well paid, they have lots of disposable income, holidays, 2 cars,a home that is full of lovely stuff but she seems discontented. If DD or myself buys something and tell her, she buys one herself, she buys designer stuff by the dozen and dresses like a teenager. She drinks like a fish and has been known to fight in bars.
She never seems to be happy, when she has dGD she buys her stuff that she doesn't need, fills her full of sweets despite being asked by DD and partner not to. I could go on forever.

Oh boy.

Before I answer, is your DDs mil happy in life generally, and is just a pain with this sort of thing?

I wouldn't be part of the other MIL's competition, I would just call in as and when it suited me and my DD, why worry what the MIL thinks?

Refuse to be drawn into it why should you feel bad at visiting/babysitting your own family. If she doesn't like it then that's her problem. Sorry but I don't 'play games', about who did what and when and for how long, life is too short.

Oh the emotional baggage that comes with Christmas/Easter/Mothering Sunday.
There is no easy answer other than alternating, designating a Sunday or weekend just before or just after or (simplest all round) declaring "a plague on both your houses" and going away.
Some people count the "turns" and make themeslves and others miserable, others go with the flow.
In this case could you have a "Mothering Saturday" with your DD?