"Faction Before Blood" was the first principle of the Factions.
When the founders of Chicago first divided society into 5 factions, they developed the principle. According to it, new initiates—on Visiting Day—were not allowed to become so saturated with their family members as it would have been regarded as betrayal of their new faction.
On Visiting Day, Eric told all initiates not to show that much affection to their relatives. In the Erudite compound, because of their hatred of Abnegation, Erudite didn't allowed Caleb's mother to enter the compound. Jeanine Matthews also showed that she believed in the system.