Can you change your will if you've been diagnosed to have an "Altered Mental State?"

My brother was diagnosed with an altered mental state in late Nov. A lady that he has dated for 4 months coerced him into changing his will from his daughter as the executor, to making her the executor. And he did. Two wks later, he passed away. Now she's running around everywhere saying she needs information because she is the executor of his will. She's trying to get all the "financial" information she can because she's wanting money. My brother had the power of attorney for our mother so his name is on her accounts. This woman is crazy and may sign my brother's name to withdraw money from my mother's account. Can she be the executor when his mental state was altered when she coerced him into signing a new will? The attorney says he has an acoustal (not sure of my spelling), that is signed and notarized, but hasn't produced it so that we can compare the signatures. He also don't have any copies of my brother's will, yet he was my brother's attorney. Can you offer any advice?