For when the Assyrian shall invade your land, you shall find the vanity of those idols to which you have trusted; and therefore shall cast them away with indignation, and be forced to seek to me for help. So this is added as an argument to persuade them to practise his counsel of turning to God. Which your own hands have made unto you for a sin; which you have made as instruments of your sin of idolatry. Or, which your sinful hands (by a common Hebraism, called hands of sin) have made for you. Or, the sin (as an idol is called, Deuteronomy 9:21) which your hands have made for you. So there is only a transposition of one word, which is very usual in the Hebrew text.
Related Commentaries of Isaiah 31:7
Isaiah 31:7
7 For in that day every man shall cast away his idols of silver, and his idols of gold, which your own hands have made unto you for a sin.