In the 1983 Christmas Special, Siegfried meets an old flame, Caroline Fisher, who has returned after living in America. They later marry and have children, as mentioned in the series 7 episode "Hampered".

What happened to Mrs Hall in all creatures great and small?

Hignett died in 1980, aged 64, shortly after the third series of All Creatures was filmed. ... Her death was written into the script of the 1983 Christmas Special, and her honour was toasted by the four remaining central characters. Her husband was buried alongside her after his death two years later at the age of 69.

Does Siegfried Farnon marry Audrey Hall?

Possibly something of a Mythology Gag, or even outright foreshadowing, on the part of the writers; Donald Sinclair, the real-life inspiration for Siegfried Farnon, was married to his second wife Audrey for over fifty years, until her death in 1995.