Sorry you are dealing with this nightmare. That’s not an affair; that’s child sexual assault. 14 is below the age of consent. Continuing sexual contact after he reached the age of consent makes this complicated, but he started out as a victim. That may be part of why his family indulges him too much, they may have feelings of guilt from not doing anything about it. Then again, they may just have been totally hands off. An abusive relationship that becomes voluntary but still involves violent arguments and the police being called is a really complicated situation that I can’t even imagine trying to figure out.
I’m sorry you have to deal with such a complicated situation and my advice is to pray and contact a really good lawyer. You may need to talk to a therapist as well. I don’t know if anyone has suggested either donating the land or the money from the sale to charity, such as a church or other well-regarded charity. If you really don’t care about the money and you don’t want them expressing bad feelings, that may be a way to handle it. You could truthfully tell them you found people in greater need than they were. Or not make that comparison if it makes them angry, I don’t know. Also a donation may make it less likely for anyone to sue or try to recover the money/land. An experienced family therapist may also need to review any wording you come up with.
So this mess is likely to cost you money

but if you are OK with that, good can come out of this situation, including maybe greater independence for Cletus.
Since they are so difficult, I’d run everything by a really good lawyer before saying/doing anything. They may suggest all communications go through a lawyer with no direct contact. And I agree that the sooner this is dealt with, the better.