See instructions in Share iCloud+ with your family - Apple Support
Note that you won't need to cancel. After she shares the plan as described in the "Share iCloud+ with everyone in your family" section of that support article and you accept as described in the "Switch from your iCloud+ plan to your family plan" section the following applies:
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.