If
a Kohen cohabits with his sister, he renders her a zonah (someone who
engaged in illicit relationship that is forbidden to her), but he does
not render her a chalalah (someone whose child looses Kohanic
sanctity). However, if he cohabits with her again, he renders her a
chalalah.
If one testifies that his son is a mamzer, and even if
both parents say this about a fetus, they are not believed, because a
man can not testify against himself. But Rabbi Yehudah says that they
are believed, because a man is believed to say “this is my first-born son,” and so also about a mamzer.
