When girls wear short pants or expose some parts of their breasts, is it a sin to look at them?
Other people may dress provocatively and with a lack of modesty but you cannot control what other people do, you can only control your own response. People who dress immodestly often do so because they want the attention that it brings them. Therefore, the answer is to not give them the attention. "I have made a covenant with my eyes; Why then should I look upon a young woman?" (Job 31:1). If you notice something that might be sexually arousing, look somewhere else.