1.If your father was the tenant then on its redevelopment into cooperative society then your father could have been inducted as member of the society if the landlord was so willing.
2.However f it is found that later his name was omitted and in his place someone else has been included then fraud of a third party is apparent.
3. However in absence of any written document to show that your father was initiated to have been inducted as member of the society it would be difficult to prove the mischief done by the step brother.
4.Howeevr if it is shown that his stepbrother has taken place of your father in respect of membership when he was no where beofre it's formation then his forgery would be apparent.
5.Sofile a suit for suit for declaration and injunction. Alternatively lodge complaint with registrar of society challenging his membership.