Yevamos 30

The Gemara says that if a man was married to an Ervah to his brother and a regular woman and Safeik divorce the Ervah, when he dies the regular woman is exempt from Yibum. Since the Ervah has a Chazuka that she's married to the brother, her Tzara has a Chazaka that she's exempt from Yibum. Tosfos points out that even if the Safeik divorce happened before he married the other woman, where I might say that she was never definitely married along with the Ervah so she never had a Chazuka to be a Tzuras Ervah, yet she's exempt from Yibum. Since the Ervah has a Chazuka that she's married to the brother, therefore we extend that Chazuka to her that we consider her a TZuras Ervah. Rabbi Chaim Smulowitz If you ever though that Tosfos was beyond you. Think again. A few more free copies of Gemara and Tosfos: Sukka 2a-11a is still available at tosfos.ecwid.com