They are telling you that you do not want to run the 12s and the 15 at the same time......Basically you usually want the same model of subs and the same size paired together.....
Find out what brand the amp is that your bro has.....and find out the model #'s for the amp and the JL subs and post em on here, these guys will be able to help you more
sounds like they would most likely be worth it......even W0's are decent subs for 300bucks with an amp.
You might not like 12's now, but I believe that if you heard a properly set up pair of 12's that you would be surprised....
sounds like maybe you are just now getting into audio or just want a loud system to show off.......you have alot to learn and I wish you good luck
Curtis