A recent academic study has shown that your relatives have not improved since Thanksgiving and are, in fact, almost entirely the same.
While there are some variables for things that have happened in the last month which could make a person more empathetic, patient and willing to compromise, there are others which make people prickly, dour and douchey, scientifically speaking. It is rare that such changes would take place in the time between Thanksgiving and Christmas, and the positives and negatives largely cancel each other out, leaving the result the same.
The good news it that, now that Christmas is over, you won’t be required to see them again until Grandpa’s birthday in April, and that’s just a dinner.