Answer:
The clause is an illegal provision.
Explanation:
An illegal provision is a clause or condition that is unenforceable by it's very nature. Kevin owns all the legal interest in the apartment. Jon has no legal interest in the apartment.
A legal interest refers to the right to legally possess and use a property. This right is enforceable under the law.
If Jon as a broker co-owns the apartment, then the clause or provision he inserted becomes legal. Given that he doesn't, the clause is void ab initio.
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