How old is your borhter. And how old is he?
Ask him a made up question about this "Keylogger" And if he replys then he might be lieing. But make it a yes or no question, and nod your head Yes just a little to lead him to say Yes..
Dont confuse him, make him think that you know what your talking about like

"Hey you got your Keylogger from zugbag right?"