Most EULA say that you may only have copy of the game running at any given time. This is why you can install steam anywhere and play it anyway, but it will force you only 1 copy running at any given time. This rule was made because of the introduction of virtual machines. Tech speaking, it's on 1 computer, but there's nothing that stops you spawning Virtual Machines with the game
So it doesnt matter who owns the game, your brother/sister/child/wife can play your games you bought - it's not "yours only"
So legally speaking, you're more than likely in the wrong (EULA prohibits 'cracking' and more than 1 running at a time).
Morally speaking, I think you are in the wrong too because you intent that someone other than yourself is going to play the game at the same time as you.
But like whatever, not like anyone really cares about it if you do that (I used to do it with my brother back when we were kids, we'd split the cost of the game and share it), it's just to prevent loopholes in piracy.
