1) your mother is co owner of flat
2) she can bequeath her 50 per cent share in flat to whom soever she so desires
3) she can also gift her share in flat to whom so ever she pleases
4) however please note that nominee is only a trustee for legal heirs
5) on your mother demise even is she nominates some one else as legal heir you would inherit her share as long as she does not make a will