If there is difference between what you feel and what your brother feels (i.e. you feel he has taken advantage of your mother and also that of your father's illness, whereas he feels he has acted legally), then you can go to Court for resolution. You may employ a good lawyer who will advise you on how to proceed, but do remember that Courts take a long time to resolve issues due to a large no. of pending cases and lawyers will demand fees, besides the tension you will have.
Please try to resolve this amicably if possible since it is your brother and mother who are involved.