One thing that might help you navigate this in the future is to keep your focus on the OTHER people in the room. And to act in a way that minimizes the drama for them.
If you can set aside your own hurt feelings and hurt pride and the sting of being disrespected, then you can focus on "how to make this exchange be low drama for the other people in the room?"
So when you're in a grouping that Chilly Relative joins, keep a mild smile, wait for them to finish, and then resume your conversation. Much the way you'd act if--oh, you and a friend went to a restaurant for lunch and the manager of the restaurant came to talk to your friend about the dinner party she was planning there next month.
You'd withdraw a little bit--enough to not be "sticking your nose in" to their conversation; you'd keep a pleasant but noncommital expression on your face; you'd wait patiently for them to finish. If it seemed they were going to take a long time, you'd say, "excuse me" to your friend and step away for a while.
Or if you were at a convention w/ a friend and their boss came up to talk business for a bit--you might wait to see if they would finish quickly, or else you'd go off somewhere else.
That's your strategy. And it might be easier to adopt it if you embrace the "separation" between you and Chilly Relative, and then focus on how to keep things low-drama for everyone else.
Oh, and if anyone brings it up, say "Chilly Relative seems to prefer not to speak with us, so we oblige him/her."
