This is weird. Although sexual relationship amongst cousins is common but real brother/sister is shocking for anyone. If it is solely because of arousal on seeing her nude, it should have ended in guilt. Nevertheless, continuing it and that too after marriage is something truly unacceptable. Moreover, they are still justifying and trying to convince you is quiet disturbing. You sound to be very sensitive on the issue (there is nothing wrong in that) and hence won’t be able to forget this incidence. In that case, it is always better to leave him rather then remaining painfully attached. This decision might be painful at the moment but on longer run it will grant you peace.