See the following from Share iCloud+ with your family - Apple Support
If a family member is already paying for their own iCloud+ plan, they can choose to switch to your plan or keep their own and still remain part of the family. When they switch to the shared family plan, they get a refund for the remainder of their personal plan. They can't keep their current plan and use the shared family plan at the same time.
For that to happen, you will need to follow the steps in the "Share iCloud+ with everyone in your family" section of that support article and she will need to follow the steps in the "Switch from your iCloud+ plan to your family plan" section.