Actually, I'm not sure about the monkeys. They're probably cool with it I guess.
I somehow doubt it, inbreeding is genetically bad for pretty much all species. I'm sure that our revulsion at the thought of shagging our own granny, stems from very powerful instincts and has little to do with "social convention".
I assume that we automatically discount the possibility of sex with any female which we have had any strong bond with during childhood – sister, mother, granny, etc. But iirc, men are attracted to women who look like their mothers did, when they themselves were toddlers – in other words, successfully fertile women.
I can see how this might cause problems when close relatives meet for the first time as adults and Genetic Sexual Attraction occurs.