Sale of property and giving posession of different property

You can file a suit for specific performance under Section 16(c) of the Specific Relief Act, 1963, to compel the third party (buyer of your sister's share) to complete the purchase by allowing you to construct the third floor, as you are entitled to the second floor. The court may issue an order for specific performance if it deems fit. You can file a partition suit under Section 44 of the Transfer of Property Act, seeking a division of the property in accordance with your respective ownership rights. This can help clarify each party's share and rights to specific portions of the property.

You may consider filing a suit to challenge the sale deed executed between your sister and the third party. Grounds for challenging the sale deed could include fraud, misrepresentation, coercion, or any other legal defect in the transaction. The court may cancel the sale deed if it finds merit in your case. 
File a suit for a declaration of ownership and possession under Section 34 of the Specific Relief Act, seeking a declaration from the court regarding your ownership rights and possession of the second floor, which would enable you to construct the third floor.

In Sri Ram Pasricha v. Jagannath (1976) Court held that a decree could be passed in favour of the plaintiff though he was not the absolute and full owner of the premises because he required the premises for his own use.
 

The facts posted disclose that the third floor is not in existence as the second floor construction is not yet completed. How could you be the owner of a non-existent floor?