In Parshat Miketz, after Pharaoh put Yosef in charge of all of Egypt we read (Breisheet 41:45): “Pharaoh gave Yosef the name Tzafnat Paneiach, and he gave him Osnat, the daughter of Poti Phera, Kohen On as a wife.”
In Breishhet 41:50-52 we read about the births of Yosef’s children: “Two sons were born to Yosef before the years of famine came. They were born to him by Osnat, the daughter of Poti Phera, Kohen On. Yosef named the first born, Menashe, ‘For God has made me forget all my trouble, and all that was in my father’s house.’ He named the second one Ephraim, ‘Because God has made me fruitful in the land of my suffering.’”
In Parshat Vayigash (46:20), the genealogy of Yaakov lists Yosef’s descendents as follows: “In the land of Egypt, sons were born to Yosef, which were born to him by Osnat, daughter of Poti Phera, Kohen On; they were Menashe and Ephraim.
From looking at the different quotes, all that we know about Osnat is that she is the daughter of Poti Phera, Kohen On, she married Yosef and had two sons, Ephraim and Menashe.
Why was Yosef comfortable marrying Poti Phera’s daughter?
Rabbeinu Bechaya, quoting the midrash, Pirkei D’Rebbi Eliezer tries to fill in the blanks as to who Osnat really was: According to the midrash, Osnat was actually the daughter that Dinah (Yaakov’s daughter) gave birth to after she was raped by Shchem, the son of Chamor. Yaakov sent her away from his home and she hid behind a bush (the name Osnat comes from the same root as sneh- bush). Chizkuni quotes the midrash in Torah Shlema which says that Yaakov hid her because his sons wanted to kill her. Yaakov tied a locket around her neck which said that she is Yaakov’s granddaughter. The angel Gavriel brought her down to Egypt. Potiphar’s wife took her in and raised her. The young women would throw their jewelry at Yosef as he walked past them. When Yosef saw Osnat’s locket, he knew that she was the right match for him. When Yaakov came down to Egypt and asked who the children were (Breisheet 48:8-9), Yosef showed him the words that he had written in the locket so many years before.
After reading these midrashim, we can now understand why Osnat is called the daughter of Poti Phera. Sine Potiphar and his wife raised her it is as if she was their own child.
We can also see why Yosef didn’t have any hesitations about marrying Osnat as he knew that she was part of Yaakov’s family.