SCARED OF WHAT HE MIGHT DO!!

My dad was an alcoholic when I was kid not to mention extremely jealous and possessive. I can remember a time my mom getting in trouble because I said a boy was cute, he accused her of looking at him and then putting the idea in my head, I was like 13 at the time. I can remember stories about how he liked it if her hair was dirty and she didn't wear makeup when she went out. Or when she started working if she wasn't home in the time he figured she should be he accused her of cheating.He used to get drunk when I was 13 and call me up and tell me what a slut I was just like my mother. At the age of 13 they divorced and boy was I thrilled, my 11 year sister was not. He used to have people spying on us after we moved and then they would report back to him. I refused to have anything to do with him. Even as an adult I was still plagued by my childhood, I would take my baby and go hide when I knew he was in the same town that I lived in, because dealing with him so frustrating and made me so angry and caused me so much anxiety. I would get so mad at just the thought of him I could hardly see straight.
 Then at the age of 22 I had my son and found the Lord and in time I was able to forgive him and we were able to have a relationship. But his recent behavior has taken me back to that 13 year old girl again. I can feel all the tightness in my muscles when I have to deal with him and the angry, resentment and anxiety building at even just the thought of dealing with him. He is not in the greatest health so I don't want to just stop having a relationship with him, but I just don't know how much longer I can compromise my mental health. My sister is going through some of the same issues as far as needing to put up boundaries but not wanting to alienate him.
 He is on his fourth marriage and they have been married for 17 years I don't think things were great this past month. But what he is doing now is borderline criminal and is emotional and mentally abusive, I am just glad he doesn't drink anymore. About a month ago My dad and his wife were having a conversation and some how it came up about men and women being friends. My dad said it doesn't work with a man and a women being friends. My step mom responded that there is nothing wrong with a man and a women being friends. Well that was it. That was all it took he was instantly suspicious of her. And every since then he has been convinced that she has a secret cell phone. He pays attention to how often she goes from the bathroom to the bedroom(which he says is constantly off and on ally day) the women is in her 70s maybe she needs to use the potty. He sneaks down the hallway and peaks through the crack in the bathroom door to see if she is using the cell phone. Pretends to sleep at night and then  sneaks out in to the kitchen and looks into the living room hoping to catch her on the cell phone that now believes she hides in her underwear. He has got a safety deposit box and  has hidden money there as well as drawn almost all of his or their savings out of their  bank and opened an account at another bank. He now waits for her to leave the house on the days that she goes out to do laundry and get groceries and then he leaves a few minutes later and follows her. He has my sister looking at bank statements and cell phone records and anything else he can think of. He told her one day that he was going to start carrying  a gun and that if he caught her with someone that that man would never break up another family. When my sister and I confronted him about that he told us that he wasn't going to do that he was only trying to scare her. That's just another way of abusing her as far as I am concerned. He is totally obsessed with proving that she is meeting someone on Tuesday and that she has another cell phone that she is texting him with. He is so lost in his own little word he doesn't see that he is pushing his children away, he doesn't realize what this is doing to us. He says he knows that he is driving his wife crazy and that he is killing any marriage they may have left but that doesn't seem to stop him. His response to everything we bring up to him is "I know". There is no getting through to him. My sister and I feel sorry for our step mom because our dad had already admitted that there is nothing that she could say that would change his thought process now. Not even Jesus himself could change this mans mind. He says he wants to stop thinking this way and then he say I will keep playing her little games as long as I can stand it. I think he is the one playing games.
 My sister and I want to talk to our step mom but she is not the most open  person to talk to and may not be the most receptive. But what we don't want is to be pawns between the two, but my dad has already put us in the middle, but we are all he has. I want so bad so tell my step mom to run and to never look back, she has no idea what she is dealing with. Even my sister's psychiatrist said that he is mentally ill and needs help. His response is if he feels he needs medication he will go get it. Well this man has need medication for more years than I can count. He won't talk on the phone with us if she is there, he won't go anywhere if she is there and he really doesn't seem happy that we are there if she is there, and when she is gone all he will discuss is her and what he suspects and he repeats himself over and over.
 I don't even want to be around him like this. I think what he is doing is disgusting. The women is basically a prisoner in her own home he finds everything she says and does suspect, even the affection she shows him. He pays attention to how and when she puts on her pjs at night. He offers to make the bed so he can leave a pillow or a blanket a certain way and then if it is different then the way he left it when he gets home he says someone has been there. This women didn't even sleep in the same bed as her previous husband.
  My sister doesn't think he would hurt her or himself, but I don't know, because I don't know where his mind is anymore. I don't trust him. My sister doesn't remember things about him when we were young and I think she has always seen him in a kinder gentler light than I have until now, but I still think she is more trusting than I am. Maybe I am not trusting enough I just feel like I need to say something to my step mom.
 I don't want to end my relationship with my dad but his behavior is taking me right back to that emotionally abusive state that I felt as a kid, and I have worked very hard to over come that.