I just want to add the note that while this does not show the collection or items in the Owners client view, the items would still be available within the Organization view of the web-vault.
So ultimately this is something that still has to has some level of “trust” that the Org Owner does not view these items if they are only intended for two separate members of a family Organization.
I believe some enchantments may be coming though under the current roadmap under Collection Permissions which will better provide for further granularity.