[divinesage]

I think the whole family needs to constantly remind him that he shouldn't gamble at all (yes, not even a single cent). If he promises that he will not gamble again, the whole family, including your mother and sister needs to take a more active role in reminding him of his promise.

It's not easy but the best way to change the behaviour of somebody is to constatly make him realise his promise and make sure he DOES NOT at any time at all gamble. The moment he does, everything goes back to square one again.

All the best for your family. If it still does not work I guess looking for relatives/friends to help him would be a good idea.