I think the issue is really small and not really worth making a point about.
I have one sister. I am married with 2 kids, she is a single mum of 1 child. We twice as many people in MY family as in HER family. Naturally.
So, for mothers day we were to buy a present and a bunch of flowers (I am also baking a cake). In my opinion it would be obvious that we split the cost in two, as it is from the daughters to the mum. She suggested throwing in the rest of the families, so the gifts were from us, AND our families. In her case also her daughter, and in my case, 3 more people. And then we split the cost accordingly. SHE pays 2/6 of the price, and I pay 4/6 of the price, as we are 4 people, and she just 2.
My mind boggles. I will of course go along with it, as I would feel embarrased to argue the point.
But, please humour the pedant in me. It is OUR mother, and mothers day, so really the cost should be split between the two of us, and who cares how many other grandkids are included on the card?
Is the cost to be split in 6 and she pays 2/6 for herself and her daughter, and I pay 4/6 for me and my family? Or do we split it in two?