Praise be to Allaah.
What you have mentioned about her suffering from retinal atrophy is not one of the faults which the fuqaha' have stated give the husband the choice of annulling the marriage. But some scholars, such as Shaykh al-Islam Ibn Taymiyah, and Ibn al-Qayyim - and Shaykh Ibn Uthaymeen who regarded it as being the correct view - narrated that every fault which puts the other spouse off, so that the purposes of marriage, namely compassion and love, cannot be achieved, mean that the spouse has the choice of annulling the marriage. (Zaad al-Ma'aad, 5/163)
Based on this, the choice of annulment is given in every case where there is a fault that affects the purposes of marriage, namely love, intimacy, bearing children, etc.
But your brother came to know of this fault after that, and he did things which indicate that he accepted that, namely continuing to be intimate with her and not hastening to annul the marriage. According to the fuqaha' this indicates that he accepts it, and that he does not have the right to annul the marriage.
But as you know, divorce is the man's right and he may divorce his wife if he thinks that he cannot live a good life with her and that he cannot feel love for her and feel at ease with her, which is the basis of marriage.
What we advise in such situations is to be patient with this wife and try to solve the problem. If her mother is the main reason for the problems, then it is better to keep a distance from her and to live in separate accommodation. If your brother lives in the same house as her or nearby, then it is sufficient to keep in touch by phone and pay brief visits, etc. If the wife's bad attitude is the reason for the problem, then your brother should check himself and how he treats his wife, for the way he treats her, beating her and calling her names, may be the reason for her bad attitude. He should seek the help of people who have experience in dealing with such problems, and try different ways of finding a solution to each problem.
Allaah says (interpretation of the meaning):
If you dislike them, it may be that you dislike a thing and Allaah brings through it a great deal of good
If it is too much for him and he finds that there is no way to solve the problem or to live peacefully with his wife, then there is no sin in divorcing her; in this case she is entitled to the mahr because the marriage has been consummated. And Allaah knows best.