The brother quickly recanted saying that he would have to check......So I'd say there was at least one year that he didn't show.
he could have been there Christmas morning and spent some of the day with everyone before taking off to Boulder somehow. I see this a likely senario because they don't remember a Christmas without him there. This may be true, but Karr's family probably is just thinking of Christmas Eve and Christmas morning, not the rest of the day.