What is the exact wording of the error message?
Are you trying to do this online, via the app or using the topup line?
Do you have your sister's member name and password?
@acmus64 Hi welcome to giffgaff and the community
You can top up another giffgaff member number try here
Another Giffgaff Number
I hope this helps
Hi @acmus64 are you sure you're have the right account username if topping up by logging into her account? Also try logging in using her mobile number.
You can't topup her account from yours.
Depending on the payment method you are using, either:
Card, PayPal or voucher:
To make sure you are in the correct account, sign in using your sisters number instead of the membername, add the password and log in...... that might do it....... 😀