What is the meaning of "our hand will not be on him"?
Redak: We should not cause his death. If we sell him, he will not die.
Alshich: We should not seize him to sell him, lest we be Chayav Misah for kidnapping and selling.
Malbim: Yehudah told the brothers to leave Yosef in the pit and distance themselves, so the Yishma'elim themselves will take him for a slave; often, someone Chayav Misah was left in a pit. We will not sell him, just we will cause him to be sold. Even so, Yehudah said "Nimkerenu", and Yosef said "Mechartem Osi 1 " (45:4).
Ha'Emek Davar: We should not strike him. The Yishma'elim can hit him, like it is normal to hit one's slave!
Ha'Emek Davar: Because they caused him to be sold, and they knew and did not protest, he said that they sold him.
Why does it say "Achinu Besarenu Hu"?
Alshich: This is another reason why we will not be liable for kidnapping and selling him; he is our brother; he is normally found with us. We exclude this from (Shemos 21:16) "v'Nimtza b'Yado" (Sanhedrin 86a).
Ha'Emek Davar: He is our Achinu (our paternal brother) and Besarenu (like our maternal brother), for Rachel and Leah were sisters. Therefore, we should not overtly pain him.