My mom has dementia and dad is starting to have memory problems. Mom thinks that she can handle things. How do we tell her we are POA?

They are together in AL and my sister and I are POA.

My dad is saying that my mom is asking what does POA mean. He is very nasty with her and has no patience and is nasty with both my sister and I.

My mom thinks that she can handle things and really she can't. How do we tell her we are POA without all the yelling and screaming.