Apple One -- Sharing outside "Family Sharing"

My wife, daughter and I share an Apple One Premier subscription by means of "Family Sharing". We would like to add my wife's sister to our plan with one of our remaining accounts. I have read the following:

"If you have the Apple One Family or Premier plan, you can use Family Sharing to share all included Apple services with up to five other family members." *

Is it possible to share Apple One Premier without adding someone to your Family Sharing? If she already has a "Family Share" can she be in two?

* Does "five other" family members mean you plus five, for a total of six? or does it mean five total?