Who is Kassim and Leila Sherban from Shikasta: Re, Colonised Planet 5: Personal, Psychological, Historical... and what is their importance?

Kassim and Leila Sherban were the orphans, who were adopted by George and Suzannah. Rachel and Suzannah, however, are charged with raising them while George is involved organizing the youth armies. The children are set up to play an important role in the post-apocalyptic period of Shikasta.