Most of my suggestions have already been offered but it occurred to me that, because your wife was the one to suggest a Dad-free Christmas and it's her brother who has demonstrated his (in my opinion) lack of filial loyalty, any response to the bro should come from her. You'd know better than we what the actual family dynamics between them might be; it's possible there is some historic dislocation between the bro and the father that has led to the son's inability to bring enthusiastic response to the prospect of having his father in the bosom of his own family over Christmas. Us outsiders mightn't see such things.

I presume her suggestion for a Christmas away with you is a 'general' desire that has gradually crept up on both of you and not driven by something more immediate and specific that couldn't have been predicted. Life can be tough. But, given her Dad's age, the "Christmas to come" without him is probably not too far off. I'd probably err on the side of avoiding rocking the boat with any detailed response to the bro; anyone familiar with folk music knows of the ability for seemingly minor family dissensions to cause unnecessary evils to the nth generation.

Best wishes.

Cheers, Rowan