If you purchase things ship them to one consistent address if it is different to your billing address. Try not to ship to multiple addresses. Also, use a card that is under the account holder's name. Using different cards and different addresses makes it really suspicious and gives them a reason to pull your account. I had to place an order and get it shipped to a different address bc I was on vacation so I called the 800 number and told them the situation, placed the order on the phone, and everything went well, without my login getting taken away either.
If anything you can always call the 800 number and ask what's up. Or you can always ask your brother to delete that login and send you a new one with a different email addy.