Technically, a registration is licensed to one person. From the license agreement:
Your single-user registration will license you to continue using your copy of mIRC.
I highly doubt Khaled has a problem with someone putting their registered mIRC on their sister's machine and registering it, although it would be nice if his sister continues to use mIRC after the 30 days, that she pay the $20. The license agreement can be ambiguous, it's probably there to mainly protect against mass abuse of mIRC.
Thank you to the original poster for being courteous enough to ask.
My own disclaimer: I cannot officially speak for Khaled on this issue, the above is a personal view.