Stingy step-grandparents

My husband's parents believe that when we got married we should have explained to all the children that they have different grandparents and that the other grandparents should not have to buy for them. My mother does not agree on this and sends equally for all four children. We asked my in-laws if this was a money issue, and the reply was that when they were married, my father-in-law's mother did not send holiday and birthday gifts to their children. They were not close because of this and maybe that is why they feel this way. I told them that they are doing the same thing to our relationship. My husband feels that if they do not buy equally, we will not join them for Christmas anymore.

This has been a big issue from the beginning, when they told us (children included) that their granddaughter is more special to them and they did not choose us as a family. We do not expect them to love the kids the same but we ask to treat them equally when they are together. Is this too much to ask?