Hi! I'll be glad to be of help with this issue.
I can imagine how frustrating this situation must be for you. You are clearly a sensible person as well as a caring sibling to your sister. So you watch as your sister has lost so much money and yet she keeps going after the same delusional hope time after time. It is so frustrating.
This is a little like the Nigerian email scam. I'm sure you've had an email or heard of it: some Nigerian government official needs your bank account information to keep half a billion dollars safe for a week and in exchange you'll get 50 million or some such amount. Right?
I read an article last year in the Wall Street Journal that reported that around 6-8 billion dollars have been lost to these scams! I may not remember the numbers exactly, but it's close to that. Remarkable, isn't it? How can people do such things?
In psychological terms we understand this as a type of addiction. Gambling is a good example. Though this is an addiction to being "gullible". If you look above at what I wrote to you at the top of my answer, you'll see the similarity to a gambling (or pornography, or any other addiction):
The person knows that they lost more than they could afford last time, they know that they're going to lose it again. That it is destructive to them. They know that this is not a healthy way to be. But that knowledge is only in a part of their "selves". There is a part of their selves that needs to feel hopeful, that needs to feel like they are going to "win", that they are going to have things work out, that things will be good for them.
This need to feel that hope gets "stuck" into the delusional and self destructive vision of the casino for a gambler, or a Nigerian email, or a man on the internet for your sister. It is a compulsive addictive behavior.
This is very unfortunate. The treatment for compulsive addictions is very intensive psychotherapy. The person has to be removed from the addictive trigger. So here it is the online site, or the internet completely, or from access to money. And therapy needs to accompany it with a psychologist who is experienced in treating compulsive addictive behavior.
Okay, I wish you the very best!
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