There is something else that adding families to families would enable though, which is better inter-family instancing. Here is an example of a problem and how nested families could fix that. Lets say 2 items from the same family collide using Physics (which is a behaviour applied to that family), and you want to destroy the one without a specific variable "can-die" and stop movement of the other but only if the other one is moving fast enough initially.
FamilyA is colliding with FamilyA
FamilyA velocity>100
FamilyA can-die=true
=
Kill FamilyA
Set Velocity FamilyA to 0
This doesnt allow us to check each item in the family against each other in the conditions, whereas if there was a nested family (which also had access to all behaviours of its parent) then we could use the tree parenting to resolve the issue. for example:
FamilyParent is colliding with FamilyChild
FamilyParent velocity>100
FamilyChild can-die=true
=
Kill FamilyChild
Set Velocity FamilyParent to 0
See how handy that would be?
It can't be done using 2 separate families as that won't allow you access to the physics behaviours from both families, where-as parenting could solve that issue