In Captain America : Civil War, is Martin Freeman's character Everett Ross, related to Thunderbolt Ross?
In the comics, there's never been a suggestion that Everett Ross is related to Thunderbolt Ross or Betty Ross.
According to a blogpost on his creator's website, when he was initially written, the writers bandied around a variety of names before settling on 'Everett K. Ross' as an homage to the character Alex P. Keaton:
The guys loved the idea, and we started hammering away at the details. The character's name was, literally, Chandler for the first couple of weeks, until I settled on a Alex P. Keaton vibe in Everett K. Ross. Most fans assumed him to be a one-off of Michael J. Fox, and Fox could certainly bring him to life, but I was writing Chandler, not Alex. I had Ross appear in KA-ZAR #17 as a warm-up of sorts, a run-through with the quick-witted, sardonic half-pint, who effortlessly got Ka-Zar off of an attempted murder charge.
Within the MCU, there's also no indication that they're related. In the canon prose novel Avengers: Infinity War - The Heroes Journey, Everett Ross refers to "the other Ross" without offering any indication that they share a personal connection.
"They're all in custody. Ross has them. You know, the other Ross". He meant General Thaddeus Ross, who had spearheaded the effort to reign in the Avengers.