If your mom is mentally capable of making decisions and directing her own financial affairs"”and it sounds as if she is"”then she should act at once to regain control of
them. That may mean making a trip to the bank and closing down her old account and opening a new one; she shouldn't be held hostage by the fact that your brother now physically holds the checkbook.
But I'm suspecting that even switching accounts won't end the potential problems that may be blooming on the family horizon. Your brother probably was well meaning when he took over your mom's checkbook. And without attempting to psychoanalyze in cyberspace, I might guess that he is the only son or the oldest sibling; it's a common pattern for them to become "helpers" even at the risk of having their help stick in the craws of other family members. It's likely that your mom felt especially muddled and vulnerable the day after your dad's death, so was unable to assert herself or make a cogent plan for her future.
But now, while your mom is able, is the time to hold a family meeting so that she can make her wishes clear. Part of the agenda should be finalizing other estate planning documents for her, such as an advance directive setting out her wishes for medical care and a durable power of attorney for finances clearly directing how her finances should be handled"”and by whom. Both documents can be slated to take effect if and when your mom no longer has the capacity to handle medical and financial matters on her own and is in need of special help. She may name anyone she wishes to act for her"”but may choose to parse out the duties among various family members.
While it's not always easy or fun to discuss these matters and get these documents in place, it can go a long way toward preventing misunderstandings and hurt feelings and to preserving family harmony.