Registered area 2 dismal. total area 6 dismal

i have a Land in Uttar Pradesh and Land Registry had been made on 2001by my step mom to me. The stamp duty had been paid only for 60x10=600sqft and boundary was given of 2400 sqft. Possession area is 2400sqft since registry time. Mutation also done on 2400sqft from 2003. 2400sqft was complete share of my step mom in that number .The area has been possessed on the basis of Boundaries, which is mentioned in sale deed.i had constructed a commercial building on 2400sqft by a loan from bank . i am paying tax of that building now my step brothers are asking for share in this building which is made by me and in possesion from 2004