Family sharing

You need to invite them to the Family Group. when they accept the invitation Family sharing is set up for them.

Family Sharing dos not grant you unrestricted access to their Photos however.

They need to actually share the photos to the shared Album for you to see them.

Set up Family Sharing - https://support.apple.com/HT201088 - "Invite people to join your family

Family Sharing of Photos - "When Family Sharing is turned on, a shared album is set up automatically in the Photos app on all family members’ devices. Then everyone can add photos, videos, and comments to the album and be notified when something new is added." http://www.apple.com/icloud/family-sharing/

The two articles I link discuss the things others have been noting.

When you invite people to the family group, ask them to be considerate when sharing photos and to be frugal with the photos they share. All members of the family group will be forced to endure all shared photos on their devices. None of the members can disable the family album. Don't dump thousands of photos into this album, but be selective, and remove older, shared photos after everybody had an opportunity to save the photos they want to keep.