Ask To Buy ( Approve what kids buy with Ask to Buy - Apple Support ) can only be turned on for members up to 17 (age varies by country - Legal - Apple Media Services - Apple ).
Above that age you will have to speak with her about these purchases. Your only other recourse is to turn off Purchase Sharing everybody in the family is responsible for their own purchases, or remove her from the family group.
Add a family member to your shared subscriptions - Apple Support
"If you're the family organizer and you don't want to pay for family members' purchases and subscriptions, you can turn off Purchase Sharing [ How to share purchases with your family - Apple Support ]. When Purchase Sharing is turned off, everyone must use their own payment method. When Purchase Sharing is turned off, family members can share subscriptions, but they can't share individual purchases like apps or books."