About two years ago, my wife begged me to let her then- 26-year-old brother move in until he could get his life straightened out. I reluctantly agreed, under certain conditions.
He had to get a job, pay rent, and work toward getting out on his own. He is no better off today, but he has cost us thousands of dollars. This has created huge relationship and financial problems. He apparently does not care how we do as long as he is taken care of.
I have threatened to kick him out several times, but they both promised that he would do better. The last time I threatened to kick him out, my wife told me that if I put him on the street she would be forced to go with him.
If I kick him out, I am without a wife, and if he continues to stay, we will all end up on the street.
Your conditions (like your threats and their promises) are just empty words. You and your wife are proving that, in the absence of a united front, your brother-in-law simply has no motivation to leave your home.
Because you're at an impasse, you could best work this out with the help of a third party. A mediator would help by getting your brother-in-law (not your wife) to agree to your reasonable requests and a time line.
Your wife would then be encouraged to agree to a move-out date, because whether or not your brother-in-law manages to get his act together, he should move out on a specific date.
You are not kicking him out - you are giving him an opportunity to be an adult. If your wife chooses to go with him, call her bluff and tell her you'll miss her.