The proposed agreement of this nature is not legally binding on the parties to the agreement, and may not be enforceable in law.

Each individual is entitled  to a legitimate share in the jointly shared property as a right.

If the joint holding is no more feasible or possible then they can go for an amicable partition after which each individual can sell their respective share of proeprty without the consent or permission of the co-sharer.

If both the shareholders agree to sell the property jointly to the prospective buyer, then they both can execute a registered sale deed jointly in favor of the buyer immediately or later without partitioning the property.

MOU can be entered into between your mother and sister son in this regard 

it can mention both would sell the land at agreed price by end of next year 

If gift deed is registered then first get your mother's name on mutation paper. For this process it will take atleast six months. Kindly provide how both your mother and aunty inherited the property and currently on whose name property belongs.

- If the said property came to your mother through gift deed , then legally she can sell her share without consent of her relative .

- For binding your cousin she can enter into MOU after mentioning the terms and conditions of selling the property . 

- However such agreement cannot be executed by the court of law . 

Let your mother and her nephew enter into a Family Settlement Deed incorporating the clauses protecting both of them.

Dear Sir/Madam,

1. You may get an agreement drafted between the mother and the cousin to ensure that neither of them back out from the decision to sell the property. 

2. Once the cousin signs the agreement, it becomes a legally binding contract and in breach of which, the cousin may be sued. 

3. However, please consult an advocate while drafting an agreement. 

Thank you,

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Yes your cousin if he is a legal heir can execute Relinquishment deed 

An agreement must be made between the parties and registered before the registrar office stating the terms and conditions of the property to be sold.

Dear Sir,

Get issue a legal notice to him and get an undertaking from him better it must be a registered document like MoU (Memorandum of Understanding) having no objection for your aunt to sell her share of property.