I would wait for her answer to your very valid question about their plans for holding off Christmas. Until she answers that they are also waiting, I would plan to open presents on Christmas Day and look forward to a second celebration with them.
I know it - we have family like this. One year, about 10 years ago, my aunt and uncle were asked to bring the mashed potatoes for Thanksgiving and told that lunch was going to start at 1PM. They showed up around 3:00PM, with potatoes that had not even been peeled, much less cooked. I thought my grandma was going to lose a gasket!
Yes, your idea was a good one. I talked to Dad and Step-Mom-they both pretty much laughed at the idea. I guess she had already talked to them about it and suggested the same things I did-skip the other Christmas or just open presents from Grandma and Grandpa and the cousins when they get there. They also didn't think it was fair to my kids to make them wait. What I might do is buy some smaller gifts to add to the pile for my kids so they end up having more to open then their cousins (that is kind of petty but oh well).