She might want time with her own children, maybe to reminisce, talk about things perhaps she only wants to discuss with them.
She may be lonely.
Your DH & siblings might be conscious of time passing, that she won't be there forever, that they want to make the most of the time they have. Mine's in her 80s and the awareness only gets more acute.
She may be one of those women who resent their sons turning attention and affection to other women! This is as we know A Thing. And they can be masters at manipulation, obligation and guilt. That needs re-educating.
She does seem to have forgotten that there are at least two other Mothers who day it will be. Regardless of how old the children are, a first Mother's Day is surely an occasion to celebrate and make a nice fuss of. Demanding ALL the attention reads as selfishness.
This is on the husbands to sort this out.
The delicate balance between pleasing mum and pleasing wife. They need to find a compromise that considers everyone - here, the social family stuff (that women usually get landed with) is now on them.
And if he does opt to bugger off and leave you doing the usual daily childcare stuff with nothing special, nothing thoughtful, nothing nice on the day ... When's Fathers Day? What will you be doing on that day? That leaves him on his own doing what you had to do on Mother's Day?