In scams like this it’s very difficult to get those that haven’t been scammed to leave it because they think they have proof that the site is legit since someone they know has been cashing out from it, without them realising that the site is only trying to make them believe that it indeed it is legit and as long as they keep paying some sets (most especially those that invested early) they’ll keep getting newbie (like your siblings) to register and when they feel they have accumulated enough money they’ll disappear into thin air.

It’s best you advise them to not refer others to the site ( although that’s the only way to earn significant amount from those sites) because you never can tell when the site will crash and if it happens that those he referred didn’t get to make their money back before it crashed he’ll be the one to pay them back i.e if he knows them.