The talk of sharing the grandchildren makes me smile.
Mother and MIL despise each other so thoroughly that all birthdays, Christmas, other holidays have to be held (in full) twice because both of them would have a full meltdown if the suggestion they share the same airspace is even mooted.
MIL refused to attend DD2's baptism because we refused to hold a completely separate lunch for "her" family afterwards. This is after she attempted to prevent me attending the lunch after DD1's baptism by inviting (off her own back) a member of her extended family who in the context of a work issue, had made written threats to kill me and had received a police warning for such.
At the wedding they had to be in separate wedding cars, on two different tables, no joint family photos, not a look exchanged between them all day.
They don't do sharing.
The effect is that neither of them can be invited to any public grandchild event now or in future. No ballet recitals, school plays, anything. They will not be there if they cannot be the only grandmother.