This should be the last page of the Christmas Eve party at Mark's parents. The next page will be the transition, and then on to Abby's where Mark plans to ask Abby to marry him. Note that I said "plans."
Here, Mark has to try to open a "Trampz" doll for his neice.
Christmas isn't complete without kids, so I've made sure that there are kids at the party. He's got no actual nieces or nephews, since he's an only child, but he does have cousins who have kids. The same is true for Abby, who does have a brother and a sister, but none of them have kids. Yet in issue #4, we saw plenty of kids at the family picnic. It's been a tradition in my family that the younger generation refers to most of the older generation as "uncle" or "aunt" and I've carried that over here.
This is all by way of explaining how Mark can be "Uncle Mark" when he's got no siblings. But having the kids refer to Mark as "Cousin Mark Once Removed" or something like that is just too clunky. Shorthand is often important in these things.