Frankly, I'm rather upset by some of the knee-jerk responses to the original question, because I came here to ask something similar, and now feel somewhat intimidated. Everyone knows there is a difference between copying and moving. The OP is talking about moving, not copying. Copying is theft, moving is not.
My situation also involves my sister, as it happens. I recently bought us both iPad 2s. She is younger and poorer than me. As a student she has trouble getting a credit card so we innocently decided that I would buy apps as needed and transfer to her. In particular, we are interested in productivity apps. After a bit of joint research I bought omnifocus, 2do and Toodledo and had a play. As agreed I had the first choice (omnifocus if you're interested!) and now want to transfer one or both of the others to her.
This is nothing to do with "theft". If I transfer to her ID, I cannot use that app any longer.
For all the sanctimonious people citing 'apple regulations' as if this was the same as natural law, I would say that I can barely think of any other commodity on this planet that I cannot sell to someone else. I could even sell my kidney if I wanted to -- though I accept that if I told the purchaser that I was hoping to retain the right to use the kidney when I wanted to, there might be problems....
I totally understand and accept that 2 users should not be allowed to share an app across 2 IDs. That would be open to abuse. But it should be possible to MOVE the use to someone else as a gift or as a sale, just as we can buy and sell the iPad itself.