I can’t tell you whether it’s a sin or not for you. It depends on many factors including the occasion, whether the person drinks/ gets drunk often, whether the person commits other sins when drunk, whether the person is of legal drinking age, whether the person or their parents or other people in the family have an alcohol problem, whether the person plans to drink in a safe environment where she will not endanger herself or others, how drunk she plans to get, etc.
There’s a big difference between my relatives getting tipsy once a year at a family wedding and a bunch of people under legal drinking age going out to a bar and getting wasted every weekend using fake IDs.
If you are wondering about the sinfulness of your behavior, go ask a priest.