A contract between two does not restrict the entire world to the terms of the contract.
Only those who are a party to the contract.
But what this does, depending upon the terms in the contract, is expose the Trump niece to a lawsuit on the grounds that she violated the terms of the contract.
I believe I read that she received a large sum of money in return for signing the contract.
If so, she will more than likely be sued for return of the money and probably will lose in court.
Contracts are a serious matter and can be enforced.
Interesting.
But how can S^S buy what the author is unable to sell?