Hi, I can answer a few of your questions
When you decide to add her you will need to change your plan to a family plan.
She will not have to purchase the same phone that you currently have, she can pick from any phone in the store.
The cost of adding a line is $10, but it will be more if she picks a smartphone because she will need to add a data plan.
Hope this helped!

This weekend im going to be adding a line on my phone for my sister but im not sure what to do, or how it works. I have a individual phone plan and im not sure if i have to change to a family plan on can i just add another line. I was also wondering if she would have to get the same phone as me (i hvae thre LG Optimus) or will they give her a choice of different phones. Also when she gets the phone how much would it cost? when i go on the
T-Mobile website it doesn't really help me out that much. If possible can someone tell me some possible phones she might be able to get.