1)
Why does it say "their sister, Serach;" and not "v'Serach Bito" (i.e. Asher's daughter), like it says "v'Es Dinah Bito" (46:15)?
1.
Tosfos ha'Shalem (1, citing Moshav Zekenim): Serach was the daughter of Asher's wife from another man. [Some texts of] Targum Onkelos (to Bamidbar 26:46) say "the name of the daughter of Asher's wife was Serach." 1
1
We must say that Asher's wife was a twin of one of the Shevatim. Serach was Yaakov's granddaughter, so she is counted. According to the opinion that the Shevatim married Kena'aniyos, why was she counted? If her father (Peloni) descended from Yaakov, she should be called Bas Peloni, and not Bas Asher! (PF).
