Buddhabelly, sorry to hear of your situation. As a mom who's gotten that collect call from a county jail at 4 in the morning that my son was arrested for an MIP and PI, I can empathize.
From a legal standpoint, at least in the state of Texas, a
first offense MIP and PI are a class C misdemeanor, which is like a speeding ticket or jaywalking offense. My son had to take an alcohol awareness class and do 30 hours of community service to have the arrest expunged from his record. This is the avenue I would have you explore with the DA if I was your mother. THEN, be aware that any subsequent arrest, the penalty and consequences increase, SO - NO MORE MIP's!!
As for the sorority, I agree that it is probably a matter of bad timing... The chapter's issues with nationals has put them under a microscope and an issue such as yours might be all it takes to bring down the house. I'm sorry for your plight, but you may be out of luck.
Good luck, and know that this is not the end of the world, you have a full life to look forward to, including the rest of your college years. Eventually, this will be a blip on the radar