Have him get a lawyer--second offenses have more serious consequences. A lawyer can help him review all his defenses and actually try to beat the case or make it smaller, lessening the consequences. These cases aren't good "do it yourself" jobs.
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There is no way anyone on this site can begin to answer your questions. A 2d DWI is serious, but particularly while he is on probation for the first. He is looking at a many months in jail; whatever his sentence is for the 1st case that was probated. He is also looking at a long, unhappy life if he doesn't stop his alcohol abuse. Get a top DWI attorney and a counselor to advise him how to handle both this case and his alcohol abuse.
Once he has a warrant out for his probation violation, he should get a bond and not have to sit in jail pending resolution of his cases. An attorney might be able to ensure that his bond amount is reasonable, especially if there's evidence he can show the judge that your brother has been proactive in getting himself into something like AA and going to meetings. Recognizing that he has a problem and addressing it will serve him well in life.
As far as the charges themselves are concerned, your brother is innocent until and unless proven guilty. A good DWI attorney will be able to assess the strengths and weaknesses of the case and tell your brother what the best way to proceed is. He can also go over the burdens of proof on the motion to revoke and the new charge, as they are different from one another. There are also consequences with DPS, and someone thorough will be able to advise him on those. That person can best protect your brother's record, and if it's someone who knows the judge and prosecutor, he can tell your brother what is likely to happen on the case.
I agree that he needs someone sooner rather than later. If you have someone lined up even before the MTR is filed, there's just extra protection for your brother there.