Twelve years after they started dating, they married. The family disapproved. Surender’s grandmother and Preeti’s mother were from the same “gotra,” or clan. Hindu clans prohibit marriage within the descendants of an unbroken male line. The Wiki page on "gotra" suggests they "only" have to go back seven generations (I don't know any of my ancestors beyond five generations and the father of one of my great-grandfather…
The traditional practice was for Brahmins to declare their exact line of descent alongside the gotra, by naming five noteworthy ancestors (pravara rishis), starting from the top level saptarishi ancestor gotra(one of seven). Some of the saptarishis famously didn't get along, and their descendants are considered incompatible matches for marriage. When considering a marriage alliance from a family with the same top level saptarishi gotra, it was considered acceptable if at least three of the five pravara rishis were different, ie, a distant branch of the family. As families lost their traditions, people identified their nodal pravara rishi as the gotra, which is how most people today know them. In addition, in ancient times the custom was for non-brahmins to state the gotra & lineage of their family priest when performing religious rituals, and ended up adopting them.