You and your brother need to file a Petition for appointment of a Guardian and a Conservator for your mother. You can request it be your brother, or the court may appoint an independent person to be her guardian.
I have no doubt your step-father will intervene and seek to be appointed for her. This is an expensive undertaking.
Is your brother up to being her guardian and conservator? I mean does he have the time to do that job.
Also, does your mother have money or insurance to pay for nurses or a nursing home?
You need to keep contact with the nurses so they can be interviewed and can be used at a hearing if necessary.
How old is your step-father?
How is his health both physically and mentally?
His health and his faculties will be an issue as well if he fights the guardianship.
Please feel free to contact me next week.