These cases are very common in various Courts of India. I can understand your concern in this regard. 
Recitals of the documents wherein it is mentioned that it was joint owned property / co owned by the parties is to be examined and also whether any specific share is mentioned therein is to be looked into. Your 2nd deceased sister's children can claim the share.
According to the 2005 modification to the Hindu Succession Act by the Supreme Court, girls are entitled to the same property as their sons. Daughters are granted birthright to their fathers’ or their ancestors’ property under this act.

Please note that Hon'ble Supreme Court has held that a co-owner of a non-partitioned property cannot transfer the entire property without getting his share determined and demarcated to bind the other co-owners.

A partition suit is a judicial action brought before a court of competent jurisdiction to divide jointly owned property. Legal Notice can be sent to the concerned parties at present through Advocate. A legal notice is a formal, written communication that informs the recipient of a legal matter or issue. It is typically sent by an individual, company, or legal representative to another party, outlining specific claims, demands, or intentions. Legal notices are often the first step in resolving disputes or enforcing legal rights before escalating matters to a court of law. It can save time, money and efforts with fruitful discussion on both side. However, if the issue is not resolved, partition suit is to be filed. Detailed discussion is required in such cases with complete details/documents.

You may contact my secretary to connect with me for clarification.

Gopal Verma

Advocate-on-Record 

Supreme Court of India

You have to execute gift deed or relinquishment deed to relinquish your share in property 

mere NOC is not sufficient for mother to sell the property 

On father demise you ,your siblings and mother inherit 50 per cent  father share in property 

By NOC your share in the property will not transfer in favour of your mother.

In any event , to stop any one of you from selling the property you can file a suit for partition and injunction.

NOC is not a document to relinquish your rights in the property.

Untill and unless you execute a registered release deed relinquishing your rights in the property your rights in the property cannot be transferred to her.

The same process is to be followed by sister and the legal heirs of your deceased sister.

Hence you can express your unwillingness for this and remain silent.

Your mother cannot sell the property without following the procedures informed above.

Your mother can sell her share alone whereas your father's share in the property devolves equally on all his legal heirs hence the mere NOC will not be recognised document to prove that you have relinquished your rights.

You can issue a legal notice demanding partition of the property and for separate possession of your share in it, subsequently you can file a suit for partition on the same grounds what you rely upon with an injunction petition to restrain her from alienating the property till the disposal of the suit, which will prevent her from taking any fraudulent action to sell the property without your knowledge or consent.

1. Property having been purchased out of money from family business, is ancestral property.
2. Mother, sisters and children of Canadian sister legally hare right to share, though sisters can relinquish their shares.
3. You can protect property by not giving NOC. There is no protection after NOC.
4. You can also issue a public notice in local newspapers warning general pubic not to purchase the property from mother  as she is only a part owner.
5. You can also obtain a stay on any transfer of property by mother.

You have your share in sane you can take injunction from court to stop any sale 

Dear Client,

Before executing any NOC, you should secure legal rights over your share in the property that came to the family business in which you have a 30% interest. Check the title documents and consider registering a caveat against the sale until it is solved. You are also advised to consult an experienced property lawyer who can advise you the way to file for a partition or restraining order to block the sale. If you are forced to sign an NOC, make sure to insert a clause in it preserving your share and rights in the property. Get all concerned parties, including your sister who has agreed to sign, to give written confirmations that will support your claim.

Hope this helps. Feel free to inquire about any legal query. 

- As per law, after the demise of your father intestate , his share in the property would be devolved upon all his legal heirs equally including your mother. 

- Further, your mother cannot sell your share without taking Relinquishment deed from you. 

- If she insist then you can claim your share after filing a Partition suit before the court. 

Precautions to Prevent Forced Sale of Property

1️⃣ Check Legal Ownership

• Since your father passed away intestate (without a will), his share in the property is inherited equally by all legal heirs (you, your mother, and your deceased sister’s children).
• Your mother can only sell her 50% share unless all heirs consent.

2️⃣ Obtain Legal Documents

• Get property documents to verify ownership.
• Apply for legal heir certificate & succession certificate to establish all heirs.

3️⃣ Your Right to Refuse NOC

• 
  Do not sign NOC if you do not agree to the sale.
• A forced NOC is invalid.

4️⃣ Your Deceased Sister’s Children's Rights

• As her legal heirs, her children (in Canada) must consent before the full property is sold.

5️⃣ Protect Your Business Share

• If property was purchased from family business funds, you may have a separate legal claim based on business ownership.

How to Protect Yourself?

✔ Refuse to sign NOC unless a fair settlement is offered.
✔ File a partition suit to legally divide shares.
✔ Send a legal notice if pressured to sign.
✔ Consult a lawyer to explore injunction/stay orders against forced sale.

For detailed, personalized advice, consider a phone consultancy. Hope you find the information helpful. You are free to contact me for further discussion. If you could spare two minutes of your time to write a review, it would be greatly appreciated and bring immense happiness to read it. Thank you. Shubham Goyal.

Sir/Madam, 

You are suggested to get the property mutated in favour of the surviving legal heirs so that NOC from all would be required to sell the said property.