I'm trying to settle my deceased parents acct.s with steifel nicolaus and trans america

My father tried to include all his investments into his trust but two of his accounts are demanding that I prove my authority to execute their affairs before they close their accts. I am the first trustee listed in their trust, but since my dad died first, my mother inherited his assets outside the trust somehow. She died only three days after my dad, who both entered hospice at the same time. My father had been her executor power of attorney in fact for over ten years because she was incapacitated with Alzheimer's. Somewhere in the fine print of the two contracts it allows them to pay to her estate outside of the trust. They are demanding that I prove my power over her affairs during those three ? days she lived longer than my dad. My father intended that all his investments were included in his trust. My mother was also not able to make any claim for payment due to her infirmity. I'm not sure why they're doing this. and I don't know which court I need to go to for this matter.