Second what that last poster said. Let your sister know that she can call you, any time, any hour of the day or night, collect if necessary, for a sobriety ride home (assuming, of course, that YOU can drive and have not been drinking). No questions asked at that time, maybe we’ll talk it over later. Check the Mothers Against Drunk Drivers web sites to see if they still have that great old parent-child agreement.
If she is underage, she risks arrest in addition to other problems, and depending on what state your college is in this could mean real trouble with the dean of students.
Beyond that, she’s growing up, she’s starting college, she needs to make her own way. Your school has a counseling office if it gets dicey. Give her the freedom.
My first legal (?) drink in college was given to me by my parish priest! --the night I made the commitment to become a Catholic (Irish whiskey, and good stuff at that). Later on I got drunk a few times in college, lived through it, graduated cum laude, finished graduate school, and have had a great life. She should too.