I disagree. They have chosen what they think is best for their own family, and the extended family has apparently gone along with them--as has the brother. They haven't refused to see the brother, just have decided not to pretend that everything is hunky-dory by treating homosexuality as normal.
The facts as stated back up my statement. The family obviously is in a position of choosing, and tensions have reached the point of the mother hanging up on her daughter. The brother, meanwhile, has (once again, it seems) "made peace" by leaving his friend at home.
As things stand now, the family tension has the appearance of being driven by Al and his wife. (Before you go off on me, note the italics. We can agree that the brother shares a hefty portion of the blame for that, but as you know from your own family fights, it's the appearance that often matters most.)
The net effect is precisely what the brother expects and desires: "intolerant Christians vs. sacrificial brother." Anger goes toward Al and wife, and sympathy goes to the brother -- which pushes the parents to a measure of acceptance for his homosexual lifestyle that they might otherwise not have had.
Showing up and being polite is not "pretending things are hunky-dory," and it's certainly not an admission that "homosexuality is normal." Everybody knows Al and wife's position on the matter.
But they're not going to make any progress by racheting up the tension. Christianity works best when it shows a peaceful face, and can show the correct way of living. A loving man and wife, with their children, on Christmas... Anybody with eyes to see will understand the fulfillment that provides. And in seeing it, they will understand the emptiness of the alternative practiced by the brother.