Is it permissible for a man to marry his sister?

Answer From The Annual Reading Schedule - Week 5 Notes For Genesis: No. Although we learn in Genesis 20:12 Sarah was Avraham's (Abraham's) sister, the daughter of his Father Terach (Terah), marrying one's sister was later outlawed in Leviticus 18:9, 18:11, 20:17, and Deuteronomy 27:22. It was necessary at some point for this to be permissible until the world had been populated enough. Of course, people coming from only one man and one woman, Adam and Chawwah (Eve), there was no choice for their children except to marry each other although being brother and sister. However, once the world was populated enough and the laws were given to the nation of Yisra'el, it became outlawed. The closest relationship I am aware of that is permissible is for persons to marry their cousin. I have not yet found a Torah command prohibiting this practice. However, please study this out for yourself and let me know if you find I am wrong. Not only have I not found a Torah command in my studies prohibiting this, but I see it being permissible after the Torah is given in Joshua 15:16-17 and Judges 1:12-13.