Carrier Adding a line, advice needed!

The only way is to add her to you plan by changing the responsibility of her line from the family plan she's on to the plan that's in your name. That's the only way to do it without incurring an ETF.

The way you suggested of adding a line and then porting her number over it will trigger an ETF on her old plan.

 

If both you and the in laws are with AT&T, just take out her existing SIM card and put it in the new iPhone from the new line. Then go buy the cheapest feature phone u can find (the Alcatel prepaid phone for $15 works perfectly), and put the sim from the new iPhone 5s in there. You'll need a nano to micro sim adapter. Then call or visit an AT&T store and have them remove the data plan on that SIM card. Yes AT&T lets u do that, which is a bit surprising since the $10/mo for just a voice plan times 24 months doesn't even pay for their subsidy to Apple.

 

Are you both on the same carrier? Knowing the answer to that is the first step to knowing what solution works best.