If both kids play varsity basketball, then both moved for athletic reasons. UIL or DEC doesn't rule on if one brother is better than the other. Allowing one brother to play just because he was only moved due to the other brother having moved, would be petty in my book. They both left a school for same school, play athletics, live under same roof. The rule should apply to both, no matter how much better one is over the other.
The parents should take that into consideration when making those decisions. Otherwise, don't transfer.