No, you would not sin by copying what other people do. These gestures are not requirements that must be performed in a certain way or with your full attention or intention. They are for your benefit. They can help you to pray. They can remind you of what is happening or what you are doing. They can be a way for you to express your love for God.
For example, the gesture of crossing our forehead, lips, and heart is to remind us that the Gospel reading we are about to hear should take effect on our mind, our speech, and our heart. Some people, as they do this gesture, silently pray “May your Word be always on my mind, on my lips, and in my heart,” or a similar prayer. I don’t always bring those words to my mind, but just envision what they mean, like a wordless prayer.