While banks and other financial institutions are known to refuse to accept powers of attorney for no good reason, in your case it sounds like the bank may have a point -- but only so far. If the power of attorney does not specify that you and your brother can each act on your own, then the bank should still honor any check signed by both of you. If it won't, you may want to contact your state agency that regulates banks to see if it will apply some pressure on your behalf.