Best answer by Mats
If the sim from your phone fits in the (unlocked)phone that your sister in law gives you, just take the sim out of your old phone and put it in the new one as David states. Then all is good. If it doesn't fit you have to buy a new sim. I just tried to buy a replacement sim online and it wants me to pick a plan as well. It seems like it's trying to add a line to my account which I don't want to do. I just want a sim card. I'm either doing something wrong or it's not possible to just buy a new sim card online. Maybe someone can chime in here on how to just buy a sim online if it's possible.
If you do get a new postpaid sim you can follow the steps in the link below to get it activated.