Hi Dinny,
Not a "chosen one" here (sorry) but just wanted to say that my sister clearly was with my mother, and she was so uncomfortable with it.
She acknowledged to me that she knew she was (great validation, as my mother would refer to any such feelings as "sibling rivalry" on my part) and couldn't understand why, and really disliked it.
To have a blatant "chosen one" is really such an unhealthy thing for a parent to do, and it seems to be disliked by the one chosen, as much as the one/s who weren't.
I'm really glad your brothers are listening to you regarding NPD, especially the brother who has now become the chosen one.
If he can see that it isn't really a compliment at all (though it may seem so at first) and that it is really the parent, using and manipulating the child, that might help him not to get caught up in it, and get emotionally hurt by your mom...
BT