By law if the one of two durable Powers of attorneys gets removed will they be notified?

Not necessarily.

Your summary is confusing. A person is not named as a Power of Attorney in a Will. In fact, this is done with two separate documents. You could have been named as Personal Representative in the Will. If you were named in a Will and it was changed, then you would not get any notice of this, in most cases. The same is true of a POA, unless you were already acting as Agent under that form. Your mother would normally be declared incapacitated by a court, although some documents provide that this can happen based on the written statement of two physicians who have examined the person.

Generally competency should be determined by the Court. As POA, you do not necessarily need to be notified although it makes sense to let you know you no longer can operate on the other's behalf. If mother was incompetent to appoint you, she is likely incompetent to appoint the sister.

First, Power of Attorney is not given via a will. The Power of attorney is a standalone document conveying specific rights to the Attorney-in-Fact. The Power of Attorney is only valid while your mother is alive. The will only can be reversed by your mother. If your mother is now incompetent, she would not have the capacity to change her will back to the way it was. You will not get any notice of any change.

If your mother gave you a durable power of attorney, then your mother has the right to revoke it. She also would have an obligation to inform you that the power has been revoked, since otherwise you would not know and could continue using it. A court could indeed declare your mother incompetent but that would only mean incompetent at the time of the court's decision, and not at the time of the execution of the power of attorney. As such, a declaration of conservatorship would not constitute revocation of a durable power of attorney. Note that much depends on what the document says as to whether it remains in force at any given time. I am simply responding to the facts as presented. You may wish to meet with a probate lawyer in your area for a short consultation to verify the validity of the power that you hold.

The will and the power of attorney are separate documents. There is no requirement under law that you receive a copy of a document revoking your appointment under a power of attorney. For certain purposes, a court order is necessary to prove incompetence.