@chrisjp
Your concerns are covered in the sub-link
Can I move an email address that belongs to another person?
The whole idea of the linking account was brought out to enable people to link their BTMail account to another person's broadband account in this type of scenario.
It was so that the person could keep the account as a BTMail Standard account rather than it being downgraded to a BTMail Basic account or having to pay for a BTMail Premium account.
When the email account is linked to another broadband account it becomes one of the eleven email accounts that the "gaining" account is able to have.
As long as the account is then "gifted" to the original owner of the email account, which it will be if the process is followed, it will mean that the original owner will have full control of the email account and the "gaining" broadband holder, in this case the OP's brother, will not be able to access or do anything with the email account except delete it if he chose to do so but not with out the original owner being made aware that that is what the brother wanted to do.
As regards the part about "knowing" the password. The gaining account holder, his brother, does not need to know this. It would only need to be known if the original owner of the email account was not able to enter the email address into the relevant part of the process.
As this process is usually done when the two parties are together it is generally not a problem but if the brother had to reveal the password, it is a simple process to change the password after the process has been completed.
