If the contract was partially performed (ie the condo was transferred, but the duplex was not, and the $20,000 was not transferred) then the party in breach can be sued for breach of contract. It is considered a partial breach if only part of the contract was complied with.
If the duplex is sold, the nephew can actually file a lawsuit requesting that the court stop the sale, in which case a lis pendens will be filed in the recorder's office, which gives notice that the property is the subject of a lawsuit. This will scare away most buyers and, if it doesn't, it certainly will deter most lenders from making a loan for that property.
The proper step would be to sue for the outstanding money, and /or to offer, and then sue, for a partition lawsuit; this is where the court will determine the parties' respective interest in the property, and allow the property to be sold, determining how the proceeds will be split based on ownership interest (ie ownership based on deed, and based on the written contract, to the extent it was performed).
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Information provided is for educational purposes only. Consultation with a personal attorney is always recommended so your particular facts may be considered. Thank you and take care.