The Return of Tanya Tucker: Featuring Brandi Carlile is The Unifying Documentary You Want to Watch with Your Opposite of the Political Spectrum Family
I think going into family gatherings with a list of compromise, fairly neutral media to screen together is a good idea. (My family loved to go to the movies together on visits.)*
I find it easiest to deal with folks like that to focus on what we have in common. I think this documentary could create some family harmony for potentially up to 1 hour and 47 minutes.