The initial Western prohibition of incest was to prevent children being born of an incestuous relationship (beneficial to society to prevent retardation and genetic defects)

In the case of adult gay twins (or same sex) any prohibition would be as legally questionable applying the US Supreme courts ruling about the Privacy of the Bedroom and the sexual behavior of consenting adults.

http://en.wikipedia.org/wiki/Lawrence_v._Texas

The majority held that intimate consensual sexual conduct was part of the liberty protected by substantive due process under the Fourteenth Amendment. Lawrence invalidated similar laws throughout the United States that criminalized sodomy between consenting adults acting in private, whatever the sex of the participants.[2]

as a layman I think Lawrence v Texas put any prohibitions on incest among adults on shaky constitutional grounds.