Goes nuts trying to find the perfect tree: Juri!
Goes overboard with the decorations: I can see both of them being picky. Juri's about aesthetic, after all!
Messes up the kitchen making cookies and other sweets: Shiori. Gotta get the frosting right!
Has to save the other from being suckered in by a kid faking a Christmas Shoes-esque sob story to get some cash: Neither of them are fooled.
Panics over buying the other the perfect gift: Poor Shiori. ;;
Insists on ugly sweaters: Juri believes she's too good for ugly sweaters, but Nanami makes both of them wear them anyway.
Insists on all the usual Christmas specials, even the terrible ones: Juri won't admit it, but there's a few cheesy specials she pretends to "tolerate."
Insists on going caroling: Shiori probably goes alone.
Leaves cookies and milk for Santa: Shiori. Juri believes Santa doesn't exist, but Shiori says she might as well humor it.
Tries to put Santa hats on the pets: Shiori in a rare bit of playfulness.
Has the relative that sends them enough fruitcakes that they're running out of room in the closet: Good ol' Student Council. Thanks Nanami.