How does the social structure of lion prides prevent incest?
It is true that African lions have recently become more prone to inbreeding, largely due to human actions (reasons include limited dispersal and at least temporarily reduced pride sizes). see here
Generally though, lions likely avoid mating with close relatives innately as most animal species do. see e.g. here
A few things about the social structures of lions (ref.):

First, many prides are not controlled by a single male. Although the males might often be related to one another.

A single male might often not be in control for long enough to mate with his grown daughters.

Females do not only mate with the leader of their pride, but mate sometimes with males from outside of the pride.