Is there a form to get permission to speak on behalf of her while she is mentally unable to make her own financial choices due to a nervous breakdown?

I was left with a ton of debt. Recently, I discovered a paperwork that states that there are unpaid death benefits by a company that lied to my mother and I after my dad died claiming we had not been covered and had nothing coming our way! My mother was forced to have to sell her 500,000 that I know my dad paid with cash. Shortly after the big market crash, it was worth only 300,000 forced from our home, collected 90,000 on the total sale of the home just to pay for medications and basic living essentials. Now, I just found death benefits unpaid. But I'm not the beneficiary but my sister. At this time, my sister is under my care because she is not able to care for herself. I also have a learning disability so I don't explain things real well. But many issues I'm finding that even the broker was lying to us and threatening to sue us if we didn't go thru with the sale of the house. Signing under duress I think. I even gave him rights to assets and things when he forced us to take the property out of the family trust.