How can the executor of a will have a parent with dementia sign property over to her and cheat other inheritors out of their share?

My dad died and as my dad was dying my sister talked him and my mother into signing their house over to her. My mother has dementia and can't remember what happened yesterday, yet my mother is signing legal paperwork?
My sister is executor and has hidden my living mothers money, has been distributing her belongings as she wants, and even took possession of my mother's wedding ring!!! My mother thinks it was lost and talks about it all of the time.
I called APS because my mother told all of us how my sister shoved her and hurt hurt. APS questioned my mother right in front of my sister and of course my mother was scared.
In the meantime my sister is running her house, as my mother said she "took everything over"
My mother feels helpless and I do as well because I have contacted many agencies and have no clue how to handle this matter. It appears that many places wanted me to bring my mother in so they can represent her, however for me to get her away from my sister would be merely impossible with her tight rein she's keeping on her and my mother is not in her right mind anyway.
How is a sibling that is watching another sibling take advantage of a parent, not following the wills wishes and making sure she possesses everything that ever belonged to your parents (while one is still alive) supposed to handle this?
My sister already told my brother and I we won't see a penny because we have not done enough for my mother. However, we are questioning her and she doesn't like it so she pushes us away, even threatening me at times. (I am the most vocal)
Any suggestions would be greatly appreciated, she is using her power as executor to financially manipulate my mother and keep everything for herself.
She already told my brother and I she will legally own the house after living with my mother for 2 years. In the meantime my mother is alone, a lot. I went there and she was walking around outside by herself at night in her night gown.
The whole situation makes me sick. She said I can take care of mom if I don't watch my step, well I would take care of my mother, but I can't move into a house that is half my sisters!!!!
Please help.