Originally posted by: MeanMeosh
well, considering that (probably) its his home network and (probably) his computer, i think he's in the ok since his mother is worried about his brother. and i think this is a better solution than no computer at all. doesnt sound like his brother is an adult yet, he is still under the care of his mother.
if his brother is going to do something that will (possibly) ruin his family and his life just to spite his mother, then i think the keylogger is justified.
edit: i also agree with richdog and michaeld, try other methods first.