Yevamot 32B

אמר רבי אבהו: מודה רבי יוסי באיסור מוסיף.

R' Abbahu said: R' Yosei concedes that a prohibition takes effect where another prohibition already exists when it is an expanded prohibition. An expanded prohibition is a prohibition that has a greater scope than the original prohibition, either because it applies with greater severity or because it applies to additional individuals. R' Yosei holds that if the second prohibition incorporates additional individuals into the list of those for whom the original item is prohibited, then it takes effect in addition to the previous prohibition that had a more limited range.

תינח היכא דנשא חי ואחר כך נשא מת ⁠— מגו דאתוסף איסור לגבי אחים, אתוסף איסור לגבי דידיה.

The Talmud asks: This works out well in cases where the living brother married his wife first, and her sister became forbidden to him as a wife’s sister, and then afterward the brother who was subsequently deceased married his wife’s sister. In that case it is possible to say: Since a prohibition was added for all the brothers, a prohibition was added for the living brother as well, and he is liable due to both prohibitions, as this prohibition is more wide-ranging than the previous one. In other words, at first this sister was permitted to the other brothers, and when the second brother married her she was rendered a brother’s wife and so she was additionally forbidden to the other brothers.

אלא היכא דנשא מת ואחר כך נשא חי, מאי איסור מוסיף איכא?

However, where the deceased brother married first and afterward the living brother married her sister, in what way is there a prohibition that adds? No additional individuals were incorporated into the prohibition as a result of the marriage of the living brother with the sister, since the prohibition here proscribing a wife’s sister applies to him alone.