Dad wants to reunite with Mom who is in end stage Alzheimer's after 31 years divorced. Can this cause decline?

Should children prevent this? My dad left my mom 31 years ago. Since then my mom has always been bitter & really hated my dad. She would rant and rave about him, and never wanted anything to do w/ him. They have never been friends, & have lived separate lives since.
My dad just got divorced from a 2nd marriage that lasted 11 yrs. He recently retired as well, and relocated from across the country. He had suddenly shown interest in living with my mom. When he first proposed this idea to my mom he did not realize she was in ES of Alzheimer's. He went to her house & moved himself in for about 2 months to eveyones surprise! My mom was diagnosed during this time, so my dad is fully aware now that she has dementia. He still wants to move in with her & " care for her" . She has surprisingly seemed open to his plans, kind of just going along. When we ask what she wants she doesn't make a decision but passively reiterates plans my dad is making, and just says she has neither told him she agrees or disagrees, but seems to be waiting for him to make the decisions for her.

We are concerned about this odd situation since my mom always hated my dad so much and they were not even friends. In fact there was some abuse on my dads part, that i witnessed, after they were divorced, so my mom had good reason to be resentful. We are surprised that my dad would want this, and feel that is prompted out of guilt and the fact that he is very lonely and aging (62, my mom is 71).

I have 4 brothers, 3 of which honestly don't care much about the situation & don't want to be burdened with my mom. One of my brothers & I have confronted our dad about this & asked him to move out of our moms apartment. He did so reluctantly & is staying in my house at the moment. He has since tried to get our other siblings to support his decision in living with my mom. He is insistent about it & has told me that I have no right to keep him away from her. Knowing their history I am concerned that this would certainly not have been acceptable to my mom, even just a couple years ago. Since he has been involved in her life she seems to be declining faster. She is confused & thinks she was never divorced. She has forgotten her friends and the people that were part of her life for the last 31 years. She also imagines my dad is seeing other women, stealing from her & seems afraid of abandonment. My dad goes along with this, letting her think they have always been married, etc, I think this is unhealthy & that this fantasy my dad is creating is not conducive to the life my mother has lived.. reality.

He has since been influencing her & sabotaging our efforts to persuade her to live with one of us, or move closer to us ( since she lives an hour away from me & 8 from my brother). He also seems controlling with her. Since he lives with me he is always here when I bring her over for a visit. When we make plans for ex. to bring my son to the park, he will prospose they go on a walk instead. ( she doesnt go, but then she wonders is shes done something wrong) He will use church to whisk her off to mass instead of allowing her to spend time with my family. I feel like he is playing games and the whole situation has me just disgusted.

Should we allow this? Is this ethical? What right does my father have to walk into my mother's life when she is vulnerable and has dementia and begin making plans for her without even consulting us? He wants us (children) to just sit back and let him take control. He has not been invloved in her life for 31 years, and my mom resented him. My brother & I don't feel this is right, nor would this be in my mother's best interest. My brother has power of atty, but we are baffled as to what to do. Any advice?