Mother incapacitated and I am medical and statutory POA. Dying brother is the alternate. How to replace him when he is gone?

Only your mom can appoint POA. If she is no longer mentally competent and you pass, the court will appoint a guardian ad litem to make decisions for her.

I would hire an elder law attorney, they can help tremendously; this situation with your POA should be easy to resolve in their hands. You can also choose a person other than your brother who would be willing to carry out the POA duties (should something happen to you), and provide instructions to that person about how to apply for guardianship of your mother.

sandjlovings: As your mother is only individual who can appoint someone to be her agent in power of attorney AND she is incapacitated, perhaps you should retain an elder law attorney.

Is there a reason you are wondering what will happen if you die? Are you ill yourself? You need to have your own will, POA and health care proxy.

Elder Law Attorney: take care of them and YOU.

in California all you need to do is write in you own writing what you want, sign it and DATE DATE DATE it. Other wise it will be next of kin who is in charge... in California it goes...
spouse, children, parents, grandchildren, siblings (in age order), grandparents, aunts/uncles, cousins.

Please refer to an Elder Law Attorney or other legal services of your choice.

If you should die and there is family available they can step up and with an attorney can get emergency guardianship or conservatorship for medical decisions. The truth is that there are few to make other than heroic measures or none in the end. And heroic measures will not be done if in the opinion of doctors they are not life sustaining, in the absence of any family. So if there is no family the MDs will make the decisions.
So if there is family let them know that brother is the second. If he is gone then whomever is family can step up and doctors will listen to them especially if they hold the advanced directives. Social workers at hospital will assist and can call a judge for temporary guardianship rights.
But a POA cannot be changed once the person who conferred it, the subject, is no longer competent to make changes. You can hire a Licensed Fiduciary, but once YOU are also gone, his or her ability to act is basically gone.
I wouldn't overworry this one. Having been in hospitals there is no way that an elder will be subjected to prolonged and onerous treatments to no avail in the absence of family demanding it be done.

Only your mother can appoint her POAs, and since she no longer can do that, then there won't be anyone to replace you or your brother. I am sorry your brother is dying. Please take care of yourself.