Planning an intervention, really need advice on what to say -what to do.

My Mom is wheel chair bound except for bathroom needs and then she has to be very carefully guided and transferred and then helped throughout with the self care, She is almost 75. She had a diagnosis of Alzheimer's but her neurologist just recently said she may not have it as her cognitive abilities are not declining as expected. She took herself off Namanda (sp?) about a year ago and hasn't had any more plaque development on the brain so I guess that is good.
However her physical abilities have been non too slowly regressing. The neurologist said it is from a brain stem stroke. One that appears on her MRI but they do not know when she had it. She is taking blood thinners and had a very brief run of PT for the stroke.
That is all I know about her medical history because my Mom and Dad will not let me or my siblings ( one older sister -involved and one older brother -not very involved) know more. I ask to go to the doctor appointments but am blown off. I think there is something more going on with my Mom because she is declining physically. I know the doctor had wanted her to do more tests but my Dad did not think they were necessary. They tried a few different tests for awhile with no results and then they just stopped. They both get yearly physicals and as my Dad tells it everything is fine but I see my Mom is declining!
Here is the big issue, though, my Dad does not want to accept any outside help. He will allow me and my sister to come and sit with my Mom for a long weekend so he can go gambling but that is about all. My Mom's needs are pretty demanding and my Dad is going to be 80 in Feb. We tell him he needs to get a part time in home caregiver to help with my Mom's baths and to give him more breaks but he is refusing! I live 4 hours away and my sister lives 3 hours away or we would come every day and help.
They live in a big old house with lots of steps and narrow halls and tiny bathrooms. Now my Mom did get my Dad to put in a stair lift a year back but it is really hard to get her in and out of the lift and the only full bathes are upstairs. I suggest to my Dad that they convert the living room that is never used onto a suite for my Mom and put on a walk in tub on that level and he thinks I am crazy. I asked if they should sell the house and downsize and he said not until he was dead would he leave. I asked again if he would please use his after care insurance to get a nurse and he says he is doing just fine. But HE IS NOT!!
He drops my Mom a lot while transferring her and she is scared when he goes to help her because of this. She begs him to get a nurse but he won't! But the worst is that he yells at her all the time! He calls here names -Crap-I am starting to cry-this is hard-it has taken me days to work up the nerve to write this- and he blames her when he drops her. He says things like " SHARON-not her real name- GOOD JOB! LOOK WHAT YOU MADE ME DO! " as she is lying on the floor. When she spills something on her lap-which is often -part of her disability, I think, he screams at her "GOOD GOING SHARON! WELL DONE!! YOU CAN JUST SIT IN IT! I AM NOT CHANGING YOU!!". I have had numerous discussions with him about how he cannot treat her like that and he says he knows, he knows but she just doesn't try hard enough. And these are the things I hear. What is he saying when I am not around? I ask my Mom all the time -do you feel safe? Does he hurt you? And she says "he yells but he takes good care of me" last time I was up , before he left for his trip, he was trying to get my Mom from her lift chair to her wheel chair and she fell and my husband heard him whisper( he whispers loud because he is very hard of hearing and refuses to get a hearing aid) "Sharon, you asshole. What is the matter with you?".
I keep telling him he needs help. He is overwhelmed but he is refusing! He has plenty of money. He even has insurance for home health care but he does not want to have to pay the 100 hour deductible. This makes me so mad because he will spend his money gambling but not on my Mom. He even told her to reuse her Depends if they are not messy!!! We told him NO WAY! I just found out he leaves her alone to go shopping. She should not be left alone! She has an alert necklace at least but still.
I love my Dad but he was never really a hands on Dad. He traveled all the time when I was a child and never really took part in raising the children. He is not a natural caregiver. Some people just are not. And he yells. He yelled at us kids growing up and now he is yelling at my Mom. But he is yelling at her WAY worse than he ever did at us kids. I wish sooo much we could move to them but there is no way we could sell our house or have my husband find a job by them. So my sister and I are planning an intervention soon to force my Dad to get help. The end line is if he doesn't Mom comes home with me. Any suggestions about how to do this? Any ideas? Please...