Your father’s second marriage is legal. The children born out of the wedlock of the second marriage are legitimate. There are four coparceners. Your two stepsisters, your father and you are the coparceners. Hence, the ancestral property shall be divided in four parts, your father and three coparceners.
The ancestral property shall devolve upon the coparceners only. Your stepmother cannot claim any share therein. This is an exclusive property because it gives coparceners a right by birth. Even father cannot take away the rights of coparcener.
After the partition, the nature of ancestral property will change. It will become the personal property of each coparcener. If your father wants to bequeath or gift his share to his two daughters, he can do so by making a Will or a gift deed.