Jamieson, Fausset and Brown on Genesis 37:31-33

31-33. they took Joseph's coat--The commission of one sin necessarily leads to another to conceal it; and the scheme of deception which the sons of Jacob planned and practised on their aged father was a necessary consequence of the atrocious crime they had perpetrated. What a wonder that their cruel sneer, "thy son's coat," and their forced efforts to comfort him, did not awaken suspicion! But extreme grief, like every other passion, is blind, and Jacob, great as his affliction was, did allow himself to indulge his sorrow more than became one who believed in the government of a supreme and all-wise Disposer.