The WTS's rule to write a Letter of Disassociation™ is like an unloaded gun being held on you, holding you and your wife hostage.
Writing such a letter is like giving them the ammunition to use that gun against you and your wife.
If you don't give them the ammunition they want and expect from you, they can't use it against you.
It's their rule. You don't have to play by their rules anymore. Live your life. Be happy. Be successful. Choose a new 'family'.