You can sell your property to any third party. Before law even verbal agreements have validity but in this case, it is a property in question and any agreement for sale has to be signed by both the parties(even digital signature is valid) along with a token money transacted as part of the agreement and part of the consideration price. In your case neither exist and mere emails are weak testament to any sale agreement between parties that too without full description of the schedule property.

Further, when it is a joint property. all parties have to agree to sell the same which is again not what happened in this case. You should be free to sell the property to anyone else.

Said email is not binding on you as you did not authorise your sibling to sell the flat 

2) original buyer cam file suit fir specific performance against your sibling 

offer was made and accepted it is contract 

however you have not authorised your sibling to sell flat and hence not binding upon you 

It is not clear whether your sibling has accepted any advance amount in pursuance of the said sale agreement or not.

If she has taken the money then immediately she has to return the same and thereafter she will have to send another email stating cancellation of the agreement.

Sale agreement itself does not create any interest in the property in favor of the buyer.

The agreement is not legally valid for various reasons including an oral agreement without any solution signature of the parties to the contract.

Both the owners are not involved in this agreement.

No consideration was paid.

Therefore you can go ahead with selling the property to a third person.

The agreement through email without each party signing it is not valid in law 

Dear client,

In India, a registered agreement is necessary for the successful sale of real estate. It's possible that the email correspondence by itself does not create a legally binding contract for the apartment's sale. An agreement must contain an offer, acceptance, consideration, and the desire to establish legal relations, according to the Indian Contract Act. In your instance, the agreement is not registered and no consideration (money) has been paid. In addition, the Transfer of Property Act stipulates that unless a registered instrument is used, a sale of immovable property is void. The purchaser does not obtain any rights over the property in the absence of such registration. The original buyer may not have the legal authority to cause issues or cause the sale to be delayed in light of these criteria, and the email exchange may not be enforceable.

Hope this helps you.

It has only validity if it’s a registered document  after payment of court fees 

yes the email agreement amounts to a contract and may be enforced by the proposed buyer by filing a specific performance suit against the seller

you have not informed whether your sibling informed the buyer that you also co-own the property and your consent/noc for the proposed sale is yet to come

if that was not done then the seller can inform the buyer that since the consent of the other co-owner is not there for the proposed sale, the same is being called off

1.  Since the seller/s have already agreed to sell the property to the buyer, the buyer may not agree for cancellation in the first instance.  

2.  However, since the email agreement was without any advance amount, the buyer may agree for cancellation.

3. Since you and your sibling want to sell the property to a different buyer, make the first buyer as consenting witness.

- As per Section 10A of the Information Technology Act, 2000, email is a legally valid mode of entering into a contract.

-  Further, the Supreme Court has accepted and reinforced the freedom that parties have to choose any method of electronic execution for entering into e- Contracts. 

- However, such contract must be signed by the parties electronically under the law. 

- Since, that contract is not signed by the joint owners , and no consideration has been paid for entering this contract , then there is no validation of such contract through email mode. 

- Hence, you can sell the property to any buyer legally , however your brother can send him a notice even by the email to cancel the contract .