1. If your goal was as you state to sell the house fast but not give it away, it should not have been listed at approximately $300,000 over what it was worth. Any serious buyer would not even want to view the property if it's that overpriced. By pricing it closer to market value, you would have many more showings which would generate offers.
2. By taking it off the market your sister has eliminated all competition and MD therefore has no sense of urgency to purchase the house. Relist it as soon as possible and continue to show it to all intersted parties until contracts are signed and the buyers deposit is in escrow.
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I personally have never heard of "delisting" a property. If it is in contract, it would not show up as an available property. If it is in "negotiations," it should still be listed and showing until a contract is signed.
Also, your attorney should be playing a more active role in enforcing " a time is of the essence clause" in all negotiations. Good Luck.