I'm not even an admin, so i know i dont have really much to say in this case... but im gonna give you some reading so so
from the view of a guy who in SWAT4 game spent 3 years to catch (get a clear proof) of one of clan leaders hacking:
EVEN IF we say that his "my brother" story is real, than as Shaya said, we would have to let both them play in our server, which gives us the risk that we just give a statement "hey lovly cheater, feel free to visit our server". I guess that noone wants it.
So even if we say that the "my brother" story is real (which in my honest opinion IS real), than i understand, that a clean player gets also banned because of some hacker... But so so i prefer them both banned, than letting a cheater in, thats the 1st thing
and the 2nd thing is that here - by keeping their IP banned - we get exacly the same effect as range bans do. Not an old example: loosrshunter - his range IP got banned, and therefore also some clean and good behaving players might also not be able to visit the server... but so so rangeban was kept. (P.S. Loosershunter of course is not a hacker, so it is not a rly simmilar case, but i used this just as an example)
So shortly saying:
1. clean huy let in, hacker let in
2. clean guy not let in, hacker not let in
I choose the 2nd option... A lesser evil
EDIT:
for some reasons which will be understood probably by ImNoob only i didnt correct it, it add it here:
in the 1st option "huy", i ment of course "guy"