Remember reading about several father/son combinations although the likelihood of that happening depends a lot on the seniority system of the airline. If any of my sons were to join my outfit, they'd have a 50% chance of starting on the same fleet and depending on career progress I might not even be on a fleet open to new hires. Some places allow down bidding, some don't.
As for husband/wife or similar combos, a lot of professionalism is required and they should definitely be free to sign off without reprimand in case of domestic troubles preceding a flight.