Ontario: waiving powrr of attorney for parent (so one sibling can handle matters)

Trying to find out the simplest way to waive power of attorney for a parent with dementia, to simplify things for family that are geographically close by. My mother-in-law has given POA to her husband, and if he were not able, to her two children jointly.

To simplify, we'd like my husband to waive the POA so his sister could be able to directly handle things. This is important because my father in law is due for a surgery soon.

Can we simply write a notarized letter? Is there a specific POA form?