She is not a "normal mom". Either this behavior is because she is being taken care of by someone, so she being "loyal" to him and his family at the expense of everyone else, or she has some psychological, emotional, or physical problems that are manifesting themselves like this.
Have you sat down with your brother for a one-on-one talk? Have you told him what you are seeing? Have you asked him to come over and bring her? Have you taken your family over to see her at your brother's? Have you asked him if he has seen a difference in her from before 2 years ago? It could be that everyone else in the family sees it, but that your brother is too close to the forest to see the trees, so to speak.
This could be something that is medical. If that's the case, a trip to the doctor, which your brother would have to set up, would be helpful.
If your brother doesn't care how you feel and isn't willing to listen or do anything to help, then just LET IT GO. If your mother goes to the old house, that will teach her a lesson. When she calls and tells you she went, tell her that she wouldn't listen when you told her over and over, so you hope she will try to listen better. I don't know what it is that she "wants" from you that she will call you for, but honestly, I would not be that kind of resource for her if all she does is only call for what she wants. If you give in to that, then you are just enabling her behavior and making her feel justified in it.
Honestly, I really think that what you will find out in a few years is that your mother has Alzheimer's or a type of dementia. Then you will understand better. Having hurt feelings from someone whose brain no longer works right is totally unproductive and does nothing but cause you pain that isn't warranted. Instead, try to get your brother to take her to a neurologist who deals with aging parents.