For free apps, sharing the link definitely works (or for paid apps that don‘t support app sharing).
But I recently wanted to buy an app that one family member already owns (to support the developer). I searched for it in the AppStore and clicked "Buy" but was then prompted with an error, that another member had already purchased this app. I had to navigate through his accounts purchases and then download the app. So exactly what you want to avoid…
I’m not sure how in-app-purchases are handled.