1) on demise of father son had 1/ 3rd share in property
2)each sister also had 1/ 3rd share in property
3) son could not transfer property in his name without sister consent
4) at time of execution sale deed did you not do due diligence
5)any lawyer would have advised you to check whether gift / relinquishment deed executed by sisters or not
6) sale deed executed must be containing ndemnity clause whrerein seller indenifies you against claims made if any
7) in the event sisters claim their share and file suit they can recover share from property in seller possession
8) sisters would also have to explain what they were doing for period of 11 years and why they did not take any proceedings to set aside sale
9) don't settle with sisters . Let them move court to claim their share in property
10 ) you are a bonafide purchaser of value and have paid full consideration to seller