You were always by his side and always helped him when he needed help to find a cure for his sister Anne. But as Ominis expected and tried to warn you for, Sebastian killed his uncle with Avada Kedavra. Anne was so sad and angry, she didn't think she could ever forgive him. Ominis didn't know what to do so when Anne wanted to turn him in he just agreed. It was two against one, because you'd never turn him in. Later he got send to Azkaban at 15 and you tried to sneak in for years. Then you finally found a way in 2 years later. You looked for his cell and walked past some prisoners when you finally found him...