My brother is one of my best friends by the way. And so it goes.
I have a sister like that. Try a discussion where you both will agree only to talk about facts, not feelings. Discuss issues, not what you think of the individual politicians or spokespeople.
And have facts at your side. For example, tax cuts have always resulted in increases to the Nat'l treasury. That is not arguable, even if the Liberals "feel" that it can't be so.
I remember a conversation I had with my brilliant MIT educated brother in 1993, during which he ranted about how evil Bush senior was. I couldn't understand why he would think that, and I argued heatedly with him. We've never been close since then.
Just go with him and use it as an exercise in reticence, an opportunity to observe the liberal wackos, and an excuse to eat popcorn.