Flat Transfer Through Settlement Deed

Dear Sir, My father & Mother (Aged 88 & 72) have flat in chennai, it is registered in both of their name. I am younger daughter, working in delhi. My father & Mother stayed along with me last 20 years in delhi, My Elder sister is living chennai, In the year 2016 my mother gone to chennai from Delhi for medical treatment (suffering from Dementia (Mentally unsound and diabetics, & cancer). During this period my elder sister had transferred the chennai flat in her name by a settlement deed without my knowledge and it was get it registered also. This was done completely done by coercive manner and Due to the continues illness of my mother died in a hospital in chennai in the year 2021. After her demises when we discuss about the properties we came to know all these things. Now my elder sister is not ready to co-operate to share the properties & its details. We have all the medical documents to prove that my mother was mentally unsound and suffering from various diseases. Kindly advice us accordingly.