Maybe the below quote, that I took from another website, may help on asserting the inhibition to be social or biological:
“The primate background to mother-son, brother-sister and father-daughter mating has long been an interesting focus for those investigating the human condition. Many aspects of group formation are considered to be adaptations to prevent serious levels of inbreeding in primates. These include offspring dispersal in monogamous forms, male emigration in multi-male groups, periodic male replacement in single male groups, and, among chimpanzees, female emigration. The most clearly recognizable incest is between mothers and sons, since in most cases fathers of offspring are not known with accuracy. Mother-son mating does occur with pre and post post-weaning offspring, both in chimpanzees and rhesus macaques (Fedigan 1982).
Nishida reports pairs of mothers and immature sons in which he says mating was often seen, but that in the only case of a mature son mounting his mother, she threw him out of the tree, and he didn’t try it again. Goodall in contrast, reported some episodes of mothers mating with mature sons (1986). This behaviour may have had other relevance than reproductive. For example it may have allowed the son the opportunity to assert himself over a female in the presence of other males. Brother-sister and father-daughter mating was not seen in Mahale because all natal females emigrated as late adolescents.
In Gombe, Fifi was seen to scream and attempt to fight off her brothers when they tried to mate with her, even though she persistently solicited mating from all the other males.
These observations of a persistent, but not universal avoidance of incest, and its variability by age suggest that the inhibitors tend to be social rather than biological factors which come into play when the relations between adult males and females are confounded by the relations between mothers and offspring.”