Maybe understandable in a naive, moronic way. This situation is all about the wife. The stepson is a tangental issue.
The wife should have agreed, way back when, that you are the parent, just as much as she is, to the stepson. That she didn’t is bad on her; that you let that situation fester for decades is bad on you. It is very divisive to a couple when the step parent is not allowed any authority over the stepchild. How is that child ever supposed to accept the step parent if they don’t have to accept regular parenting authority?
Now your wife needs to come to Jesus, so to speak, and change her tune. It may be way too late, but it still needs to be done. The stepson still needs to see you and your wife as a united front, with you having equal (and supported) authority as her.
If she is unwilling to do this, I’d seriously consider a divorce, as hard as that is. I just don’t think I’d be willing to be a partner to someone with such a twisted concept of marriage and familly. Your biological son is a problem, but that’s mitigated a bit by him being old enough to be out of the house.
I’m trying to imagine what the wife’s side of the story could be. I can’t come up with any hypothetical that would explain her behavior. Even if you had hit the kid way back when, it would still warrant a long talk with you and an agreement of how you both were to raise the child. This whole silent treatment thing is crap and you should have never stood for it after the first time it happened.
I have a stepdaughter and strived to make her as much my real daughter as possible. I never would have put up with anything from my wife that turned me into any less of a parent to her as my wife. We also have a biological daughter together. My wife and I have our disagreements about the two of them, but we always wind up with a mutual understanding and plan of action.
As far as the immediate situation with the stepson–he needs to be given an ultimatum. Give him a timeline by which he has to be out of the house, one way or another (some time to find a job, some time to find lodging, etc). Any failure to adhere to that timeline–Bye bye. I’m assuming here that the deal with him living with the bio dad will never really pan out–clearly, he doesn’t want that enough to actually do anything to accomplish it. I’m not a fan of ultimatums, either, but sometimes they’re warranted. Especially with someone that’s an adult (and has been for a while), corrupting the marriage, being a bad influence to the younger son, and bringing drugs into your house (and using them there).
The drugs thing, by the way… no way in fucking hell do I let that shit go on. Goodbye privacy. If he wants to stay there before getting his shit together before moving out, then he’s totally subject to you guys going through all of his stuff any time you want. Usually I like to address a wrongdoing with future policy, but in this instance, that policy gets instituted immediately. I’d be plowing through all of his crap and tossing the drugs, bongs, etc. Your wife, of course, needs to be 100% behind this.