How do I handle my mother who no longer recognizes my father and refuses to stay with him?

My mother has recently began showing signs of dementia, and they seem to be progressing very quickly now. She no longer recognizes my father. She knows his name and that he is her husband, but she believes something has happened to him, and that the man that is now in her home is not her husband, and she is very uncomfortable staying with him. She feels he is going to hurt her. She is calling family member and friends asking them for help and to come get her, and she is wandering off to neighbors homes begging them to let her stay with them. I don't know how to handle this. And worse, she is currently caring for a grandchild that my sister has abandoned. We have all tried to take the toddler so that we could care for him, but she becomes combative when we try to even suggest it. She takes him with her when she wanders off. I just don't know how to handle this, she is so scared she is losing her independence and claims we are all turning on her.