I did a test. So I went on my main and took out 500 from the bank which in turn took money out of both characters
to keep your money safe, don't let your cousin play on your account
There's a "add a new character" feature for a reason, he shouldn't have to make a new account just for his cousin because the system is faulty.
it isn't "faulty," they did it on purpose, and emailing them won't help
So what's the point of making a new character if you want to restart with a new bank account? yes it is a bug he's experiencing, and contacting them is the best thing to do at this point.
if you want to restart with the money, take all of your money out of the bank and keep it on the character, then your other characters will not have access to it