Property issue

Dear Experts,

Need your rxpert advice on the below issue , My mother around 55 yrs have 4 Sibblings , All the 5 have property which was given to a builder to construct  a flat which is 2 storeyed building in 2001  . The builder promised to share 5 flats which are of equal sq ft . But my mother got a flat which is the least of all and which is not approved . how  to proceed or what to do