Depending on the language of the second power of attorney, the second one may be the valid one or they may both be valid. Does the second durable power of attorney contain language revoking all prior powers of attorney? If so, then it supersedes the first document. If not, then they are both in force. In the second case, your father-in-law might want to sign a separate revocation of the original power of attorney and send it to son #1 to avoid any confusion. For more information about revoking a power of attorney, click here.