What Should I Do in This Situation?

My sister has a hickey. I have noticed and she knows that I have noticed. She is seventeen and only fifteen months older than me. I have a very good relationship with my parents and I know that if they found out she had a hickey then that would not go over to well. My sister and I are very different. My relationship is stronger with my parents then with her.I fear if I don't tell them and they find out then it will damage my relationship with my parents. She asked me to not tell our parents, but if they find out that I knew and didn't tell them then they would not trust me as much. I am worried if I tell my parents and my sister finds out that I told them then she will rat me out to the school. I am torn. Do I cover for her or tell my parents?