Quite a web there, Sad5, and so many angles from which to view...
1] This is between your husband and your father - no one else, not you, your mother, and definitely not your innocent son. Next year you should all go out for Christmas and leave your husband and father to enjoy each other's company at home... together! Something is bound to come of that little exchange - and not necessarily what you might be thinking (;))
2] Christmas is a time to put aside differences, if only for a day - engaging in the spirit if not the sentiment means holding one's tongue in all camps and letting things pass through to the keeper. Sometimes a simple "This is not the time..." comment can realign the mindset to the purpose of the gathering - there is no 'sprung floor', no ropes around the room, and no starter's bell for 'seconds out'!
3] You belong to your family (husband, wife and child) and as such this must remain your priority. While acknowledging the discontentment of your own parents, you are right to support your husband (and hopefully talk him around for the sake of your son). Keeping yourselves 'upright' and bonded will allow you to face and overcome any obstacle (even in the 'enemy's' house) - calling out what is unfair in defense of your husband and condemning poor conduct from your father because you are no longer a child and beholden to his authority. You determine how you will be treated as a guest in his house and you can, if need be, simply get up and leave- maybe try again another day until the message sinks in.
You do this FOR your parents and for your son - it is not your obligation, but your consideration that family is best when sharing the joy of new members.
