When the parents are detained by weather abroad, a man-child elder brother must take charge of his dysfunctional family’s Christmas customs.

This. But the “man-child elder brother” reference made me do a double-take. In the 2008 movie Step Brothers - they were referred to as “aimless”.

So,
“When his parent’s flight is delayed, an aimless elder brother must take charge of his dysfunctional family’s Christmas.”

Anyway, this is good stuff, a succinct logline!

I absolutely love this log line. I find this very intriguing and would definitely be interested in reading the screenplay

Thank you very much, very helpful :)

Possible Title: Christmas and other disasters

Your logline isn't bad, however, I would personalize 'the parents' with, 'his parents' or 'their parents'

Here is an attempt at a re-write:
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"When his parent's flight is delayed, a man-child elder brother must take charge of his dysfunctional family's Christmas."
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Hope that helped