What is the meaning of "Ki Yesischa Achicha"?
Rashi: It means 'When your brother incites you'. 1
Rashi (in Menachos, 39b): The word 'Hasasah' is always used in a negative sense.
What are the connotations of "Achicha ben Imecha"?
Rashi and Targum Yonasan: It actually refers to two people - 'Your paternal brother and (no need to mention 1 - Targum Yonasan) your maternal brother.
Refer to Peirush Yonasan.
Why does the Torah omit 'a father' from the list of relatives?
Rashi: A father is actually includes in the words "asher ke'Nafsh'cha".
Is this Pasuk confined to where the inciter works clandestinely?
Rashi: The Torah only inserts the word "be'Seiser" because that is the way inciters generally operate, 1 but not to preclude one who operates openly.
Why does the Torah see fit to insert the words "Asher Lo Yada'ta ... "?
Rashi: To pour scorn upon Yisrael, who drop so low, since even Nochrim do not abandon the god that their fathers handed down to them.