If the proeprty is self acquired, then your friend would be entitled to bequeath his property through the Will. However, if the daughters are minor, then they can seek maintenance out of the estate left behind by your friend.
Depnding on the religion you follow, if you are Hindu and if your parents are not alive, then you and your sister will be the only heirs of your deceased brother and will have equal share in his estate.
All children of your Ganrd father would have equal share in his estate. On death of any of the children and if there are no heirs of such children, then his /her share in the property would also devolve on his/her remaining brothers and sisters and/or their respective heirs.
You could execute a Release Deed alongwith all other heirs transferring the property to the name of your mother. The Release Deed will have to be registered.
What is the affidavit for? Your father and Uncle (if they are the only heirs of your grandfather) would be entitled to suceed to the Flat as owners thereof and the tenant would have to pay the rent to them now.