Why are some family members missing in Screen Time setup?
Here is the support article on the topic: https://support.apple.com/en-us/HT208982

The part that stands out to me is this:

"To use Screen Time with Family Sharing, you need to be the family organizer or parent/guardian in your family group, on iOS 12. Your child must be under age 18, in your family group with their own Apple ID, and on iOS 12."

They likely don't meet one of these criteria.