The master password belongs to the person who first set up the linked family of accounts. If you are not that person, you will not be able to recover that password but the original person who did set it up might be willing to pass control over to you. In many cases, that would be a parent. All they have to do is log in and change the email over to your email. Then you can change the master password.
If you are the one who originally set it all up and you don't have access to that original email account anymore, then you should contact support. Click the "Help?" icon at the top right corner of this page to get started. Hopefully they can help you get going on recovery.
Alia Misthaven