Change Apple ID old phone without password

You are welcome. If he forgot the Apple ID (iCloud) password, enter the Apple (iCloud) ID, and click Forgot Password. See if he can get a Rescue or Notification email option if necessary. Link to contact Apple Support at the end of the 2nd article.

Apple ID - Manage

Apple ID – Forgot Password  *      

If he has 2 Factor Authentication enabled, if he can't recover the password using the steps above, he will need to use Account Recovery. The Recovery Process is an automated process and the length of time it takes can vary a lot. Apple ID Support can only answer your questions. They can't intervene in or speed up the process. The article below has some suggestions to shorten the time the process takes.

Apple ID - Recover your Apple ID with two-factor authentication account recovery.   *

If you have changed your password, have you followed the steps in this article on all devices/computers? 

What to do after you change your Apple ID or password - Apple Support.  *

Ask your brother-in-law to do the steps in the article below. If you can't return the phone to him so he can do them, he can start with the 'If you no longer have your iPhone, iPad, or iPod touch' section.

Selling/Giving Away Device.  *