You don't need a marriage to create a new familial bond. You can also:
* Adopt.
* Have a child.
Is there a third non-marriage event that creates new family member?
* Adopt.
* Have a child.
Is there a third non-marriage event that creates new family member?
You develop a familial bond if and only if you have introduced an child as a family member.
Is this a necessary condition? If so, I suspect we are stuck until we revise one or both of our working definitions of family.