Can a living trust be replaced with another trust if the person was diagnosed with 1st stage Alzheimer's?

My sister had a new irrevocable trust filed to replace the current trust without our consent with my mother who was diagnosed with 1st stage Alzheimer's. My sister has full and complete control and my mother states this is what she wants, but due to her issue listed above, I don't believe she should be making those decisions. My sister has taken advantage of my mother before and I feel this current trust with all of its articles allowing no one to contest or question any of my sisters decisions is a ploy to gain control over my mother and her assets without interference.