Why does my Mom with Alzheimer's act normal when my brother is around?

Its the "Nothing Wrong With Me Show". Your brother needs an education, however that comes about. Eventually mom won't be able to act but I hope he can understand before then.
You see it and know better, no matter how much denial others have. Been there. People are afraid of that which they do not understand. He will have to come to grips with reality, hopefully sooner than later.

lostgaina, think of it this way, if you were home alone for hours on end, elderly and unable to do the things you use to enjoy when you were younger, I bet you would be napping, too. That happens when you get older, no two way around it.

Your Mom is happy and alert to see your brother because he has a whole week of things to talk about. He's not busy doing housework, cooking or laundry while he is there. She gets his full time and attention to socialize.

So that tells me maybe your Mom would benefit greatly from going to a senior center once or twice a week. That way she can be around people of her own generation :)

It's called "showtiming" and they can keep it up for a few hours, at most a day, which is why psych evals are 72 hrs long. Nobody can keep it up that long.