The London Oratory School doesn't explicitly state a formal "sibling policy" on their website in the way some schools do. However, it's generally understood that having a sibling already attending the school is viewed favorably during the admissions process. This is common for many independent schools. While it doesn't guarantee acceptance, it can be a positive factor considered alongside academic merit and other criteria.
To know for sure about their approach to sibling applications, it's best to contact the admissions office directly.