Legal Question in Wills and Trusts in Texas

No. Depending on whether or not some very specific language appears in the POA, you may be able to act for your mother (in her individual capacity) with your grandmother's estate. Do not, however, confuse your authority to act as your mother's individual agent with the authority granted to your mother by a court to act in a fiduciary capacity as co-executrix. The authority to act as a fiduciary in this context comes only from the Court. It cannot simply be passed to another by use of a power of attorney.