Can you tell the difference from depression or dementia?

MY mother had a break-down when I was 12 years old.She was hospitalized for depression and paranoia. My step-father was very abusive towards her. He would often verbally and physically abuse her. As the years past by, he would verbally abuse her and cheat on her. At the age of 68, she started to stop talking, eating, and interacting because of his lack of love. When he had another child outside of the marriage it really destroy her. Now, she does not function properly. I don't know what she is suffering from dementia, alzheimer's disease, or depression. He claims that the doctor gives her medication for her "memory". I think he is a liar and is waiting for her to die!! I went to visit her recently and she didn't recognize me. She does not interact, talk, or acknowledge what is going on around her. I feel that he is not helping her to get better. He lies and says, that he is taking her to the hospital but I feel he is lying to me. I am her daughter and I live in another borough. I feel sad because I know he doesn't love her. And want her to stay unstable. I don't know what to do legally because she is married to that creep! He has all her assets. And who knows how he is treating her behind closed doors. I don't trust him at all! If I go to court she will be place in a nursing home but first I have to prove she is being abused. I can't prove it now because I don't live with them and I only visit periodically.Another issue, she knows him and doesn't want to leave his side. And she doesn't recognize me.She doesn't trust me yet! What to do? What to do? I am clueless! I feel like if she was taking any type of medication she would be talking, interacting, and able to do things on her own. He doesn't help her and he refuses to get a home health aide or nurses aide. His income is about 6 to 8 thousand a month. He doesn't want to pay for anything for her. It brings me to tears!! I am so depress about her. I can't take good care of myself.! I feel so DEPRESS TOO!