The Four Fathers

Who was Serach's father?

1.    אשר
2.    ישוי
3.    בן של ישוי (son of Yishvi)
4.    מלכיאל

The Ramban (26:46) says the תרגום אונקלס translates
ושם בת אשר שרח,- "ושום ברת אתת אשר סרח
The name of the daughter of Osher’s wife was שרח.
(She wasn’t Osher’s daughter but his wife’s daughter)

The Ramban explains, Serach’s mother was married and
had only one child. (Serach) 
Osher married her after her husband died.*

The Techeilas Mordechai (Kur Zahav) says that she was
the daughter of Yishvi the son of Osher. When Yishvi died, Osher married his daughter in law. (איסור דאורייתא)**

A second possibility is that she was the granddaughter
of Yishvi and Osher married his son Yishvi’s daughter
in law (איסור דרבנן)

The Sefer Hayoshor says she was the daughter of 
(מלכיאל בן עילם (בן שם בן נח, 

              

*  Shu"t Chasam Sofer E,H,76 proves from here
   the Halacha in Rema Ch.M. 42:15 "adopted children
   can be called children in documents etc."
** קודם מתן תורה