Heap loads of love on them. Do the unexpected. Show them you are larger than the petty issues THEY have ....This is a time to show that you will not allow anyone to steal your day, to steal the joy from this season.
Avoid topics that are button pushers for them. Decide if it is more important to win a conversation before you begin it. If you know that certain topics push them over the edge and make you want to s c r e a m, avoid them.
Sadly there are those who no matter what you say it is the wrong thing and will provoke an unwanted exchange. You know that already so learn to bite your tongue.
Who cares if they think they are winning whatever the 'contest' is.....you know that you have chosen to have a lovely holiday even if those poor souls have not.