I've got two brothers in California who are hard boiled libs. You're exactly right about how politics ranks higher than blood family for them.
When my dad passed last August, all the siblings gathered at my one lib brother's house after the service. At some point during the evening, he asked me (in front of everyone) "What do you think about Trump?"
Without hesitation, I said, "He's a good man."
You would have thought I cursed his family or something. He jumped out of his chair, screaming, "Go home! Go home!"
I would have been glad to, but I was 1,500 miles away from home, with no personal transport that evening. What got me worst of all, is that it didn't matter to him that we had just buried our father just hours before.
Sorry for your loss and the scene caused by your brother.
After the election of Trump, Prager did a show on this where mostly leftists were disowning family over Trump.