What would you do about a parent who you think has early onset Alzheimer's dementia?

Here is the situation, so bare with me, its kind of a long story. My Mother, who I temporarily reside with with along with her 87 year old boyfriend, is 65 . They are not married. He pays for food , but no rent and occasionally fixes her car and drives her everywhere. I moved in to stay with her temporarily this past year after the sudden death of my Sister,in the cabin adjoining their house. Lately, I have noticed a very extreme decline in her mental abilities,she is forgetful and such. Here also is the problem: I am moving back to my home state in August and I feel very bad about leaving her in this state. After all, what if something should happen to her very old boyfriend? I cant get her to take a test too determine if anything is wrong with her. She is also very head-strong and stubborn and somewhat simple-minded. We aren't very close, so I cant convince her to take any test. However,she is my Mother and of course I care about her well-being. I have 3 brothers and a Sister and they all(except one) don't care and/or are indifferent to the problem. My sister is estranged from her,the other brothers are indifferent. One brother has taken notice, but like me feels helpless also and does not know what to do. They all live in a different State. I want to and need to resume my old life,yet I feel like something will happen to her when I am not here. Her boyfriend treats her okay, but like I said they aren't married and he hasn't always pulled his weight(like I said he doesn't pay rent here. and I don't know if its just a relationship of conveyance for him and if he will con her when I leave. She has no mortgage and has some money( I think) left from her previous husband who passed away. I just feel like I don't trust him and I know that he knows of her mental decline and he has agreed with me that she acts a little crazy at times. I want her to move back home with me,although it would be tough on me at first until a situation can be in place,however she is bent on staying here with her boyfriend. Am I wrong in wanting him to go away and wanting her to move back near her children under the circumstances? He makes her happy(I guess) How would you handle this problem, if she were your parent?Any comments ,or suggestions? Also,she is blind in one eye and sort of emotionally unstable after my Sister's death