I think that, just like the scene were they are smoking and they get caught by Mummy Holmes, they wanted to show that family is where Mycroft allows himself to drop the facade and the 'perfection' and actually says what he thinks. He does that when he's with Sherlock in other situations as well (bickering at the palace - 'I'd let you', bickering in the flat after the explosion -'Don't make me order you').
I think it does make sense as we all feel more at ease and feel like we can 'let lose' a little more when we are with close relatives who have known us all our lives, and the Christmas scene was particularly funny because it was so over the top (and OF COURSE someone like Mycroft would hate Christmas )

Neat comments… and agree with both!  The brothers get a little immature, sure, and agree about the 'facade' with family, but at same time, it is oddly different for him in particular.  The one place/group he feels he can be annoyed?
I don't have much else to contribute, but for some reason, you reminded me of the perfect scene (other than the cottage one), that ties into that.  When John is first introduced to Mycroft, the brothers bicker a bit right off the bat, being both humorous, and showing us that that's par for the course with them. Years down the road, it's kinda awesome in retrospect that we get a xmas family scene, because what did Mycroft quip to John?  'Imagine the Christmas dinners'.  And Sherlock retorts at his accusations of being whiny/petty that he's not the one who upset Mummy!        (Hmmm… implying Mycroft always did, being like that, or Sherlock being a little blind to his effect again and they both did) 

They're both overgrown kids.