Hi LK KONG,
Your sister can check if she needs to file a tax return here:
Check if you need to send a Self Assessment tax return.
If she has now received the UTR but does not meet the criteria she can submit a request to close the Self Assessment record.
If she needs to file the tax return as the 2022/23 tax return will be issued late she will have 3 months a week to file the tax return before late filing penalties charged.
The original payment date 31 January 2024 would still apply so if a payment is due and paid late interest and late payment penalties may be charged. If she receives a penalty she can appeal:
Check when to appeal a Self Assessment penalty for late filing or late payment
Thank you.