It's not your fault that the person commited suicide, it's the said person's fault. Don't beat yourself up about something like that. It was their choice to commit a horrible act not yours.
Anniversary is the right word, and it's terrible. Everyone feels guilty when something like that happens. A girl in my county committed suicide a few months back, and her best friend told me that maybe if she'd just given her that hug on Friday that she didn't have time to do, or sat next to her at lunch that one day, it would have been different. It's so sad, especially when you add in the bit about the older brother. I wish these things didn't happen. Try not to beat yourself up about it. :(