>>women could sniff out genetic differences in potential mates<<

The ensconcing, in common language, of a supposed direct reciprocal and 'inter-personal' relationship of gene to bearer and bearer to gene is an interesting phenomenon. At the level of a woman smelling a potential mate, I would say she was able to detect her own feelings of pleasure or displeasure. And I would even take it the next step and say that this functions to detect 'histocompatibility.' But to say 'she can detect the genes..' seems a stretch of the popular imagination, probably arising in a professional term of art or classroom shorthand.

>>i think this too is part of a natural selection against incest<<

Similarly, I would probably tend more towards a more immediate explanation -- that her newly mature offspring represent competition for scarce resources such as food. (There is a theory that human children remain small a long time because it makes them unable to compete with adults in food gathering). This, especially when there are overtones of social taboo.

--as usual, I speak in both cases as a layperson.