Legal heir certificate doubt

As the property is your father's they have no right in it.

that is a debatable issue. however, if your uncles and aunts are cooperative, get their signatures. if not, then see if the buyer is insisting for those signatures. if buyer is insisting and your uncles and aunts are not cooperative, then you will have to go into litigation because technically, any property which comes to a hindu female becomes her absolute property. 

The property share of your grandmother left behind intestate, shall devolve upon her legal representatives and their consent for transfer / sale is mandatory.

The ideal is if you can negotiate pre-trial and cooperate with relatives... 

if the property belongs to your grand mother and her legal heirs will have to sign. If the property belongs to your father(self acquired) the LR's of your grand mother need not sign.

Yes, all the legal heirs of your grandmother need to sign the sale including the siblings of your late father unless the property is in Kerala where Section 15 of the Hindu Succession Act has been amended as follows -

Amendment of section 15.-In the Hindu Succession Act, 1956 (Central Act 30 of 1956), in section 15, after clause (b) of sub-section (2), the following clause shall be inseried. namely:- "(c) any property inherited by a female Hindu from her pre-deceased son shall devolve, not upon the other heirs referred to in sub-section (l) in the order specified therein, but upon the heirs of the pre-deceased son from whom she inherited the property." 

Incorporating the Kerala Amendment, Section 15 of the Act reads as follows:

Section 15.   General rules of succession in the case of female Hindus.

(1) The property of a female Hindu dying intestate shall devolve according to the rules set out in section 16,

 

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(a) firstly, upon the sons and daughters (including the children of any pre-deceased son or daughter) and the husband;

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(b) secondly, upon the heirs of the husband;

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(c) thirdly, upon the mother and father;

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(d) fourthly, upon the heirs of the father; and

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(e) lastly, upon the heirs of the mother.

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(2) Notwithstanding anything contained in sub-section (1),

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(a) any property inherited by a female Hindu from her father or mother shall devolve, in the absence of any son or daughter of the deceased (including the children of any pre-deceased son or daughter) not upon the other heirs referred in sub-section (1) in the order specified therein, but upon the heirs of the father; and

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(b) any property inherited by a female Hindu from her husband or from her father-in-law shall devolve, in the absence of any son or daughter of the deceased (including the children of any pre-deceased son or daughter) not upon the other heirs referred to in sub-section (1) in the order specified therein, but upon the heirs of the husband.

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STATE AMENDMENT

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Kerala.--

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Amendment of section 15.--In the Hindu Succession Act, 1956 (Central Act 30 of 1956), in section 15, after clause (b) of sub-section (2), the following clause shall be inserted, namely:--

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"(c)" any property inherited by a female Hindu from her pre-deceased son shall devolve, not upon the other heirs referred to in sub-section (1) in the order specified therein, but upon the heirs of the predeceased son from whom she inherited the property."

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[vide Kerala Act 17 of 2016, sec. 2].

As per your query, let me tell you that your father though have her mother's in his legal heir certificate, but after the death of his mother (i.e. your grandmother), it does not mean your grandmother's rest of the legal heirs will aslo have rights over the property.

Basically, after the death of your father the property had four rightful and legal owners i.e. you, your sister's and your grandmother. However, as you've mentioned above in your query that your grandmother had also passed away, the rightful and legal ownership is distributed between you and your sister's only, and not to any other person.

Thus, in order to sell/mortgage/lease the property, you must get the agreement signed by your sister's, since they along with you are the rightful owner and enjoy equal rights in the property, and except them you need not to get the agreement signed by anyone else.

Hope It Helps!

Regards
Kevin M. Paul