Can a sibling change a Power of attorney without notifying the other sibling?

Your sibling cannot change your mother's POA or will only your mother can do that. If your mother was mentally competent at the time, then the changes would most likely be valid a person can change her POA and will at any time she wants, and is not obligated to inform anyone such as another child. Note this also assumes that the POA and will are valid on their face--that is, they are properly signed, witnessed, notarized, etc. If they are not, then they may be invalid simply for not being executed properly, and if invalid, the prior POA and will would still be in effect.
There are some grounds to invalidate even the properly executed will POA of a mentally competent person, such as "undue influence" the will and POA were changed in accordance with the desires of, or to benefit, someone who had disproportionate power or influence over the other person, such as the caregiver for a mentally competent but physically frail or disabled person, but these can be hard--not impossible, but hard--to show, since the law presumes that a will, etc. created by a mentally competent person is valid.
If your mother was not mentally competent at the time the new will and POA were created, they are not valid. 
Under these circumstances, you should consult in detail with an elder law attorney. He or she can explain your options, such as how to gain access to your mother to verify that she executed these instruments of her own free will and that they are validly executed or what to do if you suspect that you mother may not have been competent when she created them. The facts are critical, so you need to discuss the facts in detail with a lawyer.