rights with joint poa with brother over mother

The problem may be due to miscommunication on the part of the nursing home employees.  Although they claim to only be able to send one notice, you should contact the manager or person in charge of the nursing home to attempt to rectify this situation.  Since your brother is the primary contact for the nursing home, and although you also have power of attorney, the nursing home may still want an (unnecessary) authorization from your brother for you to be included on the list of people receiving  information about your mother.  If they still claim to only be able to send one notice, the nursing home should at least provide you with the information on your mother by telephone.

If these methods do not correct the problem, you may want to consider a lawsuit against the nursing home in which you would seek an injunction to compel the nursing home to provide you with information about your mother based on your power of attorney.  This would be very costly and therefore should only be a last resort.