An attorney's signature must also be witnessed by someone aged 18 or older but can't be the donor. Attorney's can witness each other's signature, and your certificate provider can be a witness for the donor and attorneys.
Witnesses – Your signature has to be witnessed by either a lawyer, a qualified legal executive with at least one year's experience (they must be working for and directly supervised by a lawyer), or someone from a trustee company like Public Trust.
There is no general rule that says a family member or spouse cannot witness a document, as long as they are not a party to the agreement and will not benefit from it in some way.
The Power of Attorney needs to be signed by the principal, giving the agent authority to act on his/her behalf. The principal's signature has to be co-signed by at least one witness to confirm that it was indeed the principal signing the document.