I am not trying to make excuses for anyone but if your sister has information to share and some stuff may shock you, you may have a better understanding of what has happened.
I had a sexual relationship with my twin sister for a number of years.
I did not mention his religion to say that it's allowed in his religion or it is because of his religion, no it's not allowed in any religion and in any culture, it's allowed neither in his religion and nor in my religion and culture, there's no connection between people's wrong doings and the religion.