1. your mother can always change the nomination during her lifetime
2. a nomination does not mean that the nominee will become the owner of the flat after the demise of original member when society transfers the share certificate to the name of nominee
3. a nominee only stands as a trustee on behalf of all the legal heirs of original member
4. therefore when society transfers shares to the nominee's name that does not mean that the nominee becomes owner of the flat
5. despite the nomination, your and your sister's share in the flat after the demise of your mother will be equal i.e. 50:50, unless your mother has not disposed of her flat during her lifetime by sale, or has made a bequest of her flat disturbing the normal line of succession, or if there are more legal heirs than two, in which case the flat will devolve on all the surviving legal heirs equally