Hi
1. if there is a time clause in the first agreement with the first purchaser, and the purchaser didn't honor it then the agreement registered will be void as per the terms. such situation there should have a Notice reply or a notice send to the purchaser demanding cancellation of the sale agreement .
your registration sale agreement without cancelling the first one is illegal.
The possession was with you at the time of the first sale agreement is not a legal issue , but now the title as well as possession is with you after the second sale agreement with you and nephew
2. did your nephew return the money to the purchaser or offered to do so.
3. The sale agreement with the first person has to be reviewed to give a proper advise.
4. Court will look into the merit of the case and will pass order, so you should take efforts to prove that his agreement has been violated as he did not comply the conditions of it within the time frame.whenever there is such delay or violation of contract by the purchaser seller has to mandatory send a notice and call off the deal officially and demand for the cancellation of the agreement
5.In your case your nephew has not acted and failed to honour the contract by himself.
6. in the court go for a settlement