"Line"? As in her new cell phone number?
You add Apple IDs to Family sharing, not phone numbers or specific devices.
If her devices are using an Apple ID that is already part of family sharing, then there's nothing you need to do. changing the phone number on her iPhone does not affect Family Sharing in any way.
If her email address is already part of Family Sharing, then you cannot add it again, you also cannot create another Apple ID with it.
What you seem to want to do, should not be need to be done. As long as the phone with the new number is logged in to the Apple ID , it continues to access Family Sharing normally.