iPhone Keeps Asking for Other User iTunes Store Creds - Which Content is Causing this?
I think you're on the right track.  If you checked every app and know for sure that it was downloaded from your iCloud account, then it's probably the music.  

You mentioned you used Family Sharing.  If you're the organizer, you can remove her from the group or disband the family group.  According to Apple:


  When you leave Family Sharing, your Apple ID is removed from the family group, and you stop sharing music, movies, TV shows, books, and apps with any remaining family members. In addition, any photos, calendars, and reminders shared by the family group will be removed from your devices. If you chose to share your location, you'll stop sharing your location with other family members, and your devices will be removed from the family Find My iPhone device list.
  
  Any content that your family shared with you isn't automatically removed from your device. You can purchase it again or remove it to free up space on your device. If you downloaded an app from a family member’s purchase history and made In-App Purchases, you'll need to purchase the app yourself to access your In-App Purchases.
  
  Any DRM-protected music, movies, TV shows, books, or apps that you previously downloaded from other family member's collections will no longer be usable, and other family members won't be able to use content downloaded from your collection.


The family organizer can remove anyone over 13 from the family group at any time.

Go to Settings &gt; [your name] &gt; Family Sharing, tap the name of the family member you want to remove, then tap Remove. If you're using iOS 10.2 or earlier, go to Settings &gt; iCloud &gt; Family.



Can the family organizer leave or disband a family group?

Yes. If the family organizer stops Family Sharing, it removes all family members from the family group at once.
Go to Settings &gt; [your name] &gt; Family Sharing, tap your name as the organizer, then tap Stop Family Sharing.  If you're using iOS 10.2 or earlier, go to Settings &gt; iCloud &gt; Family.