There is no inherent right to be able to 'share' apps, even though you have paid for the right to use one. When you 'buy' an app, you are actually not buying the app, you're buying a license to use that app subject to certain conditions. You can share apps on iOS because Homeshare (or whatever it's called) is a licensing agreement created by Apple to allow people to do this under defined terms and conditions. AFAIK, Microsoft has no equivalent licensing arrangement for WP apps (nor does Google with Android apps). Licensing for Windows Phone apps is for use on a single account. It may be possible to transfer the license to another account, but then the license for the original device would probably no longer be valid.
Originally Posted by Jrexxx
Your analogy with DVDs, CDs is flawed. You have every right to lend her a DVD/CD - while she has it you can't use it yourself. If instead you copied the DVD/CD and gave her the copy to watch or listen to, you are in fact breaking the law by breaching copyright. So with an app, the equivalent would be for you to allow your sister to use your copy of the licensed app, but while she's using it, you can't. That's not what you are proposing.
None of this is new. Single user software licenses have been around as long as the PC.
So whether you consider it stealing or not, it is (practically speaking) stealing if the terms of the license you have purchased don't allow you to do it.
The question of whether the ability to share apps is a good thing or not, or something that companies should allow because it's good PR (as Apple must believe), is a completely different question.