Very commonly used and inconspicuous excuse for getting out of pretty much anything. One can replace the word 'brother' with any relative that would make sense. Possible substitutes include but are not limited to:
Sister, Cousin, Nephew, Niece, etc.
(Guy 1) Hey Guy 2, wanna hang out?
(Guy 2) One sec... Nah, I have to take my brother to baseball practice.
(Guy 1) Hey Guy 2, wanna see a movie with some friends?
(Guy 2) Can't tonight, I have to take my brother to baseball practice.
(Guy 1) Hey Guy 2, wanna play some football today?
(Guy 2) Sorry man, I can't play today. I have to take my brother to baseball practice.
Fogey/fogy /fougi/ sl. (early 18C+, orig. Scot) old-fashioned, stuck-in-the mud.
Person with old fashioned ideas which he is unwilling to change: Come to the disco and stop being such an old fogey!
You think me an old fogeyand an old tory, his thoughtful voice said. I saw three generations since O’Connel’s time. I remember the famine. Do you know that the orange lodges agitated for repeal of the union twenty years before O’Connel did or before the prelates of your communion denounced him as a demagogue? You fenians forget some things. (James Joyce, Ulysses. Penguin Books,1992. p. 38)