What a thought provoking exercise.
The answers I came up with are NOT ideal. I think it would be like little sister and DH and I go it alone. The addition of SIL (and his dad) really changed our family dynamics. We have prepped for the adult children coming here-but can you ever have enough?
The neighbors are a possibility. They both have great skills and would be assets but they also have adult children/grandbabies they are planning for.
This requires more thought. But, if everyone (family) converged in a SHTF situation thank goodness we have a psychologist in the family.