I presume your are not the family sharing Organizer?
The payment method for Family Sharing is automatic:
1. If anybody in the family buys or subscribes to something, Apple first attempts to charge the item to that individual's Apple Account (Apple ID) balance, if any.
2. I If a family member does not have enough personal Apple Account balance, any excess will be charged to the Family Organizer's primary payment method (usually a card of some kind).
Note that if the Organizer ends up paying for an item that that person (your mom) will receive the receipt for the item purchased, even if you initiated it. To avoid this you need to ensure you have sufficient Apple ID balance in your account to fully cover the costs of items.
Apple doesn't say what happens if you turn off purchase sharing yourself but you are still a Family Sharing member. Maybe if that is the case the payment rules still apply?
Ref:
- Family purchases and payments - https://support.apple.com/HT201079
- How App Store and iTunes Store purchases are billed - https://support.apple.com/HT201359
- Check your Apple ID balance - https://support.apple.com/HT202359
- Change, add, or remove Apple ID payment methods - https://support.apple.com/HT201266