Added on January 31, 2018:

When I said that I didn’t know who had written on the wall, I was actually sure my little sister was the culprit. But since she was always blamed for everything, even things she didn’t do, I wanted to protect her and feign a poor memory.
BUT! JUST RECENTLY! My older sister informed me that the permanent-marker-wielding-potty mouth was, in fact, ME! She remembers watching me do it. My mind is blown.