I'm searching for innovative approach in order to explain discimimation versus non family members, going beyond intergroup relations theory (ingroup vs. outgroup, that is family members vs. non family ones) within family firms.
There is little research on this topic, beside your own! Three more recent papers:
Mustafa, M., Ramos, H.M. and Man, T.W.Y., 2015. Linking psychological ownership to employee extra-role behaviours in small overseas Chinese family businesses: Does family status matter?. Journal of Entrepreneurship in Emerging Economies, 7(2), pp.129-147.
Ramos, H.M., Man, T.W.Y., Mustafa, M. and Ng, Z.Z., 2014. Psychological ownership in small family firms: Family and non-family employees’ work attitudes and behaviours. Journal of Family Business Strategy, 5(3), pp.300-311.
Erdem, F. and Atsan, N., 2015. Trust Based Relationships between Family Members and Long-Term Employees of Family-Owned SMEs. International Business Research, 8(4), p.223.
But if what you are looking for is an innovative theoretical framework, I would venture to read J.P. Rushton's genetic similarity theory: he basically argues that our generosity, empathy, and altruism are directly proportional to the percentage of genes we share with others: we care more about our own children and relatives than about our fellow countrymen, and we care more about our fellow countrymen than about people living at the other end of the world:
Article Ethnic nationalism, evolutionary psychology and Genetic Simi...