Fiduciary dilemma and legal implications

[SusanB679898]

My mother (87) and my sister (64) live together. Mom has significant cognitive decline and my sister is severely mentally ill. I was durable, medical and financial POA for both. 

My sister has been receiving SSDI for years and my mom was her fiduciary.

My father passed away a little over 2 years ago. I was in the process of settling my dad's estate when I was informed by my sister they had a new trust in which I had been relieved of all duties. My sister now had those duties. Reasons given for my removal were my desire to place mom in a nursing home and my sister in a psychiatric facility. At some point, my brother (who I am estranged) became my sister's fiduciary to maintain SSDI.

I have many concerns and worries over this and no legal authority.

Is it legal for my sister to have fiduciary responsibilities over our mother when said sister is under fiduciary care herself?

Trust me when I say I have left out 99% of the drama.