The unregistered MOU is not legally valid and cannot be enforced in law.

In general, unregistered documents relating to immovable property cannot be enforced in law.

The Supreme Court has ruled that an unregistered agreement to sell or general power of attorney cannot be used to transfer the title of an immovable property.

Section 49 of the Registration Act, 1908 states that unregistered documents relating to immovable property cannot be used as evidence, unless they are required to be registered by the Transfer of Property Act, 1882.

Your lawyer is correct 

un stamped and un registered MOU for division of immovable properties is in admissible in evidence 

3) your father in law ought to have filed police complaint against his siblings for cheating criminal breach of trust , fabrication of documents 

Ownership if property cannot be created without registered deed. Any thing on letter head has no value. No need to bother. 

Contact for  same, I may provide you through thorough legal consultancy and case laws in the matter. 

thanks for your appreciation 

[R. Hemalatha v. Kashthuri, 2023 SCC OnLine SC 381, decided on [deleted]]...

https://www.scconline.com/blog/post/2023/04/15/unregistered-agreement-to-sell-shall-be-admissible-in-evidence-in-suit-for-specific-performance-supreme-court-legal-research-legal-news-updates/

-  As per Section 10 of the Indian Contract Act, all agreements are contracts if they are made by the free consent of the parties competent to contract, for a lawful consideration and with a lawful object and are not expressly declared to be void.

- Further, a MOU is a statement of understanding between two or more parties which when made has no enforceability in the eyes of law; as such an agreement has no intention to create a legal bond between such persons. 

- But, when an MOU has been made as per section 10 of the Indian contract Act, then it is a valid agreement.

- Further, no stamp duty is required on MOU, but if the MOU for an agreement related to immovable property worth more than Rs. 100/- , then it is mandatory to be registered and stamped.

- Since, the said MOU is not registered from the Registrar , then it has no legal value in the eye of law, and the Court cannot admit the same in evidence. 

There isn't much information about a Supreme Court judgment that makes an unregistered memorandum of understanding (MOU) invalid, but here's some related information about unregistered agreements and the Supreme court.

The un-registered MOU does not create any charge or interest for the agreement holder in respect of the property as per Section 54 of T.P.Act. When there is no charge or interest created in favour of the MOU holder, Section 91 and 92 of T.P.Act cannot be of any assistance to the opponent.

DRAT decision in M/S Super Sales Corporation vs The Debt Recovery Tribunal on 8 October, 2012

Dear Client,

There are several legal grounds that can be used to counter the MOU that your father-in-law’s siblings have formulated. First, the MOU is signed without stamp and registration hence cannot be invocated under the Indian Stamp Act. Thus the MOU may be held to come under Section 14(d) of the Specific Relief Act and therefore cannot be specifically enforced. If there were omission of consideration or if the MOU was prepared and signed with fraudulent intention (for example, using letterhead with prior consent) than such considerations would make the MOU legally void under the Indian Contract Act, 1872. Your lawyer should file an application to dismiss the suit based on these argument and further rebut the admissibility of the MOU as evidence. Similarly, relevant case laws like Royal Orchids vs. Kulbir Singh Kohli & Anr. where the court stated that the effect of such agreements cannot be achieved where the agreement fails to meet the basic legal formalities.

Hope this answer solves your problem.