You have probably already told them, and it did not work. This page is a different approach.
Being told you are being deceived is being told you have been foolish. Most people defend against that, and the more directly they are told, the harder they defend.
Scammers build for this. They often warn the person in advance that their family will try to stop them, so your arguing becomes evidence that the scammer was right.
So the approach is not to assert. It is to ask questions they have to answer themselves.
Ask: how did they first contact you?
Ask: has anyone told you to keep this from your family?
Ask: what happens if you say you need a week to think about it?
Ask: have you been asked to move money to keep it safe?
Ask: would you be willing to call the bank on the number on your card, together, just to rule it out?
If money is moving and the questions are not landing, stop persuading and start protecting. Call their bank yourself and say you believe a relative is being scammed. Banks have a process for this and you do not need their permission to raise it.
Send yourself this page so you can forward it, or read it later when you are not standing in a shop.
Email it to myself