Have I committed a sin by waiting for my sister in a cafeteria that has immodestly dressed women, even though I tried to sit far away from them?

The presence of men and women in one place is not forbidden if each party adheres to the regulations of Islamic law, such as hijab (veil), modesty, lowering the gaze, and avoiding khalwa (private seclusion of a man and a woman). However, if there is physical contact between the sexes, or ostentatious display, or a soft manner of speech, then this mixing becomes forbidden. Accordingly, your avoidance of sitting with women who display themselves immodestly and your lowering of your gaze is not considered a sin, and safety lies in avoiding sitting in such places.