SEND ME A SHIP AND I’LL TELL YOU:
Who wears the ugly Christmas sweaters: Neph would definitely be the one where an ugly Christmas sweater more often, but I'm sure she'd pretty easily convince Takeru to do the same.
Who picks out the holiday movies and who makes the hot cocoa: Neph would pick out the holiday movies while Takeru made hot coca for them to share.
Who starts the snowball fight: We already know Takeru has a habit of starting snowball fights. XD
Who drags the other under the mistletoe: Again, Takeru.
Who decorates the house: Neph mainly. Takeru wouldn't have the patience for it, but he'd help out where he could.
Who hangs up the ornaments on the tree: I feel like this is something they would do together, even if Neph has to persuade Takeru to help out every now and then.
Who cooks Christmas dinner: Neph
Who invites the other to sing a Christmas duet: Probably Neph, I can't really imagine Takeru suddenly wanting to burst into song.
If they have any holiday traditions: To be determined....
Who would start a food fight during baking: Takeru XD
Who would get drunk off of eggnog: Neither? Of the two it would probably be more likely for Takeru to drink too much, but I can't really see him doing that often unless someone like Set came around and he had a drinking buddy.