Family app sharing asks for payment again

I’m trying to download an app that a family member bought. I downloaded it by clicking on my initial at the top right of the App Store and viewing her apps, and there was a download icon next to the purchase. I clicked it and it downloaded the app. When I went in to the app, it offers the free trial or wants me to choose how to pay for it. But I thought the point of this was to not have to pay for it again… What what am I doing wrong?