Lynchmob, families can be so difficult to navigate!
Part of the problem here is, if I read it correctly, your step-mother's attempts to make everything "right."
Sometimes, we need to utilize "tough love" to make people face up to their own responsibilites. Because if they know that there is a "safety net" there, i.e., family that will bail them out, they may never learn.
You might wish to try and expalain this to your step-mom. And do not let guilt force you into protecting your brother from a lesson he needs to learn. That's called enabling. Sorry if that sounds harsh, but it's true.
I recommend letting your brother know you are there for him, but are not going to bail him out. Guilt shouldn't be a motivator. If you can help financially and wish to do so for reasons unrelated to what he has done for you in the past, go ahead.
As far as the DUI's go, this is serious if it is his second offence. This one you should steer completely clear of. I had a DUI once and a night in jail and the associated fines cured me of driving drunk. Seems like he has yet to learn that lesson. Let him learn it before he kills or hurts someone.
"I finally have my heart!"
To the perps: Don't worry about me coming after you. But you damn well better watch out for God! "Vengeance is mine", saith the Lord