How can my four siblings and I contribute financially to my mom in her retirement in a way that protects her from any of us exerting control over her?

My mother is finally retiring from her time as a public school teacher - she is nearing the age of 83. She has a pension but no savings, and there will be a shortfall for her monthly expenses. How can my four siblings and I establish an agreement and process for contributing money to her on a monthly basis? We want to protect Mom, and ourselves, from a situation in which any one sibling could exert power over her etc (we do have one sibling who has untreated paranoia, which adds a complication and adds to our concern - he has left voice recorders, motion detectors, etc around her property without permission and is convinced that multiple people from his past are out to get him, including parents of childhood friends, bosses, etc). Nothing we can do about his mental state at this point and Mom's financial security needs to be tended to regardless. How can we best support Mom and avoid potential future conflicts/nightmare scenarios?