Their lives and mine have become miserable. Earlier this year, she was hospitalized with a bacterial infection, which triggered DIC and then she became septic. She lost both of her hands due to this illness. The amputation surgery took around 9 hours. Her dementia rapidly increased after it. They argue constantly. She has been verbally and physically abusive to him. She says he has been verbally abusive to her. She says she doesn't like him and she is pretty sure she doesn't love him anymore. He says he is not going to live this way any longer. He is going to leave it to me.
Curious, how much physical therapy did your Mom get after the amputation, and how much emotional therapy did she get? You did mention her dementia rapidly increased.
As for your step-father I can understand him being totally frustrated with the whole situation. Some men [and women] just cannot cope with a spouse who is no longer the same person they had first married. I don't know if joint counseling would help, or even if your Mom can gain anything from that.
Are you living with your mother and stepfather? What kind of support are they getting at home?
Seek outside help urgently, is all I can say. You ALL must need it.
Before this became so critical, did you all get along pretty well? If so, keeping hold of how things were before might give you an idea of the kind of normality you need help getting back to, and therefore what sorts of support will be most useful. I'm so sorry for what you're going through.