The simple act of showing cleavage is not a sin, no. But it does cause others to stumble. There lies the sin. You are causing others to stumble.
Some people have very high sexual tendencies and minds. If I have been good in the Lord all day and controlled my sexual thoughts and thought about God all day, and then I see you walking in the room with your stuff hanging out, I start to think bad thoughts. Guess what? That's your fault, not mine, and while I'll ask for forgiveness, you have a part to play in it as well.