Yes, it may not be the mother's fault, I just said it could be, it's stepdad's job to see. Kids make bad choices inspite of what they are taught, remember the prodigal son.
The simple facts/ or what I'd do;
1. get a gun safe, a very secure safe. Do it today, what if he doesn't get convicted. Hung jury, bad prosecution, "good" defense attorney.
2. get another very secure ammo storage, preferable hidden too.
3. while he's in jail (hope it's 5 years, not out in 18 mo. for "good behavior") sell all his stuff, yard sale, unless he's got a job and paid for all his belonging they're not legally his. That way there will be very little left for step 5.
4. change all the locks while he's locked up, a simple preventative measure, and consider a good alarm.
5. when he's let out all his stuff is on the driveway, boxed up, like a good parent would do. He doesn't come in the house but you can help him move.
6. Let him know that when he shows he'll be a responsible person he'll be alowed to enter your home to visit.
7. Do everything a good parent would do. Help him find a place to live, help him get a job, help him not become isolated or mixed in with the wrong people. The prison will punish him, your job will be to help rehabilitate him.
8. Don't be a fool, don't enable bad behavior, and don't believe lies.
I can't even start to understand all the heart ache your family has gone through. I hope you stepson will grow-up and become a respectable man.