daf bit: Yevamot 32

(Today's daf is 33.)

The mishna teaches: if two brothers were married to two sisters and one of the brothers died, and then after the wife of the second brother died, the surviving brother is forbidden to marry the surviving sister. Why? Because she was forbidden to him for one moment, no matter how briefly (after the first brother's death and before the first sister's death), and so she is forbidden to him forever. In the g'mara the rabbis say that if he had relations with her anyway he is doubly guilty, on account of "his wife's sister" and "his brother's wife". There is then a long discussion of this, continuing through most of today's daf. (32a)

(It seems like she would be forbidden to him from the time his brother married her, which is more than a "moment", but I'm using the (translation of the) mishna's language here.)