UPS policy in the district I retired from was that a relative could work in the same building as long as neither of these folks had any direct or indirect management authority over the other relative. So in a large hub it is possible for a relatives to work in close proximity to each other. If you have a position with district responsibility it is unlikely you could have a relative working in that district.
I had two situations where other managers had their sons working for me. There was no influence on me to treat them any different. Actually, it was a benefit because if they screwed up (and one did) his father kicked hist butt back in line.
Nepotism in reverse!
The biggest concern I would have is a management person who might be to "anxious" or worried to treat the relative of a higher up the same as any other employee.