The BIG issue of incest is females getting impregnated by a male member of their family (brother, father) because that results in horrible birth defects because of double-doses of bad genetic information.
Humans have 23 pairs of chromosomes. Each pair gets half it's information from the father, and the other half from the mother. Everybody (except me) has some defective genes. However, the good genes dominate the bad ones, so the bad ones generally get ignored. BUT.. if your mother and father both have the same bad genes.. the odds of the baby getting BOTH parts of a chromosome with defective genetic information go up exponentially. This is why royalty and isolated communities are plagued with a much higher incidence of all sorts of problems such as hemophilia, retardation, missing or deformed limbs, dwarfism, etc.
That is the reason why incest is illegal, and was illegal before people even understood genetics and how babies born of close relatives had so many problems.
With that said, there is no physiological reason why gay men or lesbian women should not engage in incest. In fact, I have read that 60% of identical twins have engaged in sex with each other at least once.
There is the psychological aspect of it, including being shunned by other people finding out that twins Billy and Bobby are diddling each other, but there are no physiological reasons.