My older sibling is POA over our mother's care (88) and showing signs of forgetfulness. How can we change to another competent POA for her?

Flygirl, make an appointment with the Attorney who drew up the Power of Attorney, or an Elder Law Attorney, to create a more current POA.

I would suggest that 2 names be placed on the POA, just in case the first person no longer can or wants to continue as POA.

Your mother can easily make a change to whoever else she chooses as long as she’s competent to do so. My dad changed to me doing it all, POA and executor duties, from a sibling of mine. It was an easy change with his lawyer

Only your mother can change her PoA. If she only has mild cognitive/memory impairment an elder law attorney will privately interview her to see if she can comprehend what she has asked to do (create a new PoA). If so, then it can be done, but don't delay in doing this.

If your mother is too far gone, then your family may have to seek guardianship for your mother. I'm not aware of another alternative but consulting with an elder law attorney in this case would be strongly recommended and money well spent.