Evolutionary transitions to and from complex social behaviour appear linked to female multiple mating (polyandry). Specifically, female polyandry decreases the relatedness between family members which has fundamental implications for the costs and benefits of cooperation. Here we examine the role that female polyandry plays in influencing social dynamics within a group of family living lizards, the Egernia group. Focusing on Egernia whitii and utilising 10 years of data from a long term study, we show that males and females live in long-term stable monogamous pair bonds and parents look after their offspring. However, we also show that females are polyandrous with approximately 30% of offspring sired by males outside the pair bond (extra-pair offspring) and that female polyandry has significant ramifications for both parent-offspring and sibling-sibling interactions. We then explore the evolutionary consequences of these effects for the stability of the family group more generally and provide a framework for connecting these within population effects to the diversification in family living that we observe right across the Egernia group.