One mistake that people make in regard to politics, and indeed in regard to religion, is that they overlook the extent to which factors other than intellect lead people to make their commitments. For example, in the 19th century, if you were an Irish-American, you were almost certainly a Democrat. If you were a WASP, you were almost certainly a Republican.
This remains true today. People who want to assemble political coalitions can take some things for granted. White southerners and blacks will be in opposite camps. WASPs and white southerners will generally be in opposite camps.
The best predictor of one’s political allegiance is one’s parents’ allegiance.
Your older relative likely has visceral identification with the political party in which he grew up. Life-long identification is very difficult to overcome. At some point, one has to weigh the costs of trying against the benefits.