Welcome to SR. Your brother is facing a consequence of his actions. Sometimes, recovery just comes too late. After 4 years of dealing with him and his alcoholism, she's just had enough. When a person reaches the point where they've had enough, they are just done, regardless of whether the alcoholic enters recovery or not. Like you said...she's heard it all before. There are no guarantees that he won't go back to drinking and she's reached a point where it doesn't really matter anymore. She's done. It happens more often than you might think.
I hope your brother will continue on with recovery and find a program to help him. Just not drinking rarely works in the long term.