Then you have nothing to transfer.
Just follow these steps to create a family group and invite her to the group:
https://support.apple.com/en-us/HT201088
This will not affect her Apple Music library in any way or delete anything, but once this is done and if you have a family subscription on your account, she can disable her own Apple Music subscription and will still be able to access the service from her account. Your accounts and libraries will remain separate but she will be able to use your family subscription.
Not as well that after you do that all iTunes purchases made from her Apple ID (apps, music, movies, subscriptions, etc) will be charged to your credit card.