need solution for property disputes among mother & two sons?

Dear Client,
Your query appears to be cloudy to some extent as to how your mother-in-law got the entire ownership of the property on the death of your father-in-law unless he executed a deed of gift or a will in favour of her. So, presuming that she acquired the ownership of the flat based on one of either mode of transfer, in the given situation, unless she agreed to transfer the property by way of gift to her both sons, a relinquishment deed of one son relinquishing his right to property to other would not be considered as a valid transfer of right in the absence of ownership of the property as on the date of relinquishing the right to said property. So, considering the potential risks and complexities involved in the offer of your brother-in-law, your concern appears to be justified and reasonable. Reach out to an Advocate for guidance and steps.

Dear Client,
After the death of your father-in-law, all the legal heirs (i.e. mother-in-law and Children) will have equal share in the property. If the property needs to be completely transferred to your mother-in-law then the children need to execute a gift deed or relinquishment deed in her favor. In the present scenario, the son willing to relinquish his rights and receive 10 lakhs in return may do so by executing a relinquishment deed and mention about the consideration amount taken in lieu of the same. Further, when the property is entirely transferred to your mother-in-law, she can transfer it to your husband by executing a gift deed. However, on her death the property will be inherited by all her legal heirs and your husband being one of the Class I legal heirs under the Hindu Succession Act, is entitled to the property share on priority. If your husband wants to receive the entire property then it is advisable to get a gift deed of the property executed by your mother-in-law with her free consent during her lifetime. If required, you can consult an advocate for advice and assistance.
Hope it helps.
Thank you.

Dear madam,
It seems your brother in law is playing with you. You can get relinquishment deed from both by paying Rs. 10 lakh to each and get joint or individual relinquishment deed from your mother in law and brother in law to avoid future complications. If your mother in law stay in the house then take rental agreement from her.