Gabrielle had been smoking and reading in her room when she had felt a vague sense that something was wrong. At first, she thought she was becoming bored with the book in question, but it soon became clear that the book (Strapping Lads And Their Wands: The Saga of the Leather Pants) was no less compelling, it was her that was off.
After a few minutes of trying to puzzle out what was wrong, Gabrielle sat up, stubbed out her cigarette, and pulled something on that was more appropriate for travelling the halls. She couldn't explain why, even to herself, but she felt compelled to visit Fleur, to make sure her older sister was all right.
Creeping around the hallways was no great challenge, after having done it for the rest of the year, and Gabrielle knew the hallways weren't often patrolled anyway. Bellatrix was usually off having her sadistic fun, and the rest of the teachers actually worked or slept this late, and so Gabrielle was very shortly at her sister's door.
Normally Gabrielle would have knocked politely, but something told her if she did, she would never find out why she had been led to Fleur's room, and so she opened the door easily (most security charms that Fleur cast recognized Gabrielle, and vice-versa). What Gabrielle saw when she opened the door, though, made her wish for a second that she had bothered to knock.
Fleur was trying to hold back tears, staring at the mark Bellatrix had left on her skin -Bellatrix had marred her sister's flawless body, and even the thought of it made Gabrielle clench her jaw. Gabrielle rushed forward, all thoughts of decorum forgotten as she wrapped her arms tightly around her sister.