My Mother wants to know if she can add or remove someone from her investment money as a POD.

Not sure I follow you regarding the investments. Do you mean your Mom wants to change her beneficiaries on the investments [stocks, money markets, etc]? If yes, one can do that on-line or by calling the investment broker.

Now if you mean your mother wants to remove your sister as financial Power of Attorney and assign someone else, I not sure that can be done now since your mother is under guardianship. Your profile says your mother has Alzheimer's/Dementia.... an attorney might consider your mother not able to understand, thus unable to sign a new Power of Attorney.