Hi
Issue notices to the buyer and your sister. Both of you have inherited the property in equal shares and hence both of you can only sell the property jointly to a third party. Also in a family inheritence either of the parties have the first right of refusal(i.e you can choose to buy out your sister's share or vice versa) before selling to a third party. In your case both your invaluable rights have been trampled.hence you need to issue notices and file suit .