How do I join my family if they already have Apple Music

Your Mom needs to invite you to to join as outlined here:

From https://support.apple.com/HT201088 :

Invite people to join your family

As long as you both use iOS 8 and later or OS X Yosemite and later, you

can add anyone who has an Apple ID to your family. If you have children,

use these steps to create their Apple ID and add them to your group. Use the steps below to add someone who already has an Apple ID.

On your iPhone, iPad, or iPod touch:

1. Go to Settings > [your name] > Family Sharing. If you're using
2. iOS 10.2 or earlier, go to Settings > iCloud > Family.
3. Tap Add Family Member.
4. Enter your family member's name or email address and follow the onscreen instructions.
5. If you’re using iOS 11 or later, choose whether you’d like to send
6. an invitation via Messages or to invite them in person. Then follow the
7. onscreen instructions.

On your Mac:

1. Choose Apple menu () > System Preferences, then click iCloud.
2. Click Manage Family.
3. Click +, then follow the onscreen instructions.

If your family member is with you, they can enter their Apple ID

password on your device to accept the invitation. You can also send them

an invitation, and they can accept from their device. If you have

multiple Apple IDs in iTunes, you can invite each of your accounts to

the group, so you can share purchases from your other Apple IDs with

your family. 

See if your family member accepted the invitation



After you send the invitation, you can check its status under the person's name.

On your iPhone, iPad, or iPod touch:

Go to Settings > [your name] > Family Sharing. If you're

using iOS 10.2 or earlier, go to Settings > iCloud > Family. 

On your Mac:

Choose Apple menu () > System Preferences > iCloud > Manage Family. 

Then select that person's name to see the status of the invitation.

If you need to resend the invitation, select Resend Invitation.