How does one help care for a 94-year-old father when only sibling is bipolar?

How does one help care for a 94-year-old father when only sibling is bipolar, and has major mood swings? Can put on "normal" act, so people around him think the rest of us are crazy. He is manipulative, was enabled by mom who passed just a month ago, is semi-supported by father, and the lifelong estrangement issues go on. Dysfunction Junction. Both of us have dad's best interest at heart, but I am blocked at every suggestion and ridiculed by he and the caregivers (he has become close personal friends with them). All I want is to be able to communicate with him and my dad over best decisions for dad's wellbeing and safety. Dad has no mobility in lower legs. Needs ADL care and help with any ambulation-- walker or transport chair. Huge fall risk. I am trying to be the rational one, but feel I am being pushed out by manipulative brother and caregivers (private group-- not agency).
I want my dad's last years to be peaceful, safe, and for him to be allowed to grieve my mom in peace. I literaly cannot stand to be in the same room with my brother. Help! Am I the crazy one?