I agree with telling the police. let me add just a couple tidbits.
1) Tell the police NOW. Also tell them who knows, and about the secret file and where it is that proves this.
2) Police can only investigate a crime. So if he has "urges" but hasn't acted, it can be a hard sell.
3) Tell the sister. Tell her NOW. Do not assume she knows. Tell her what you saw and why and how. Especially if you don't care about the hall, think of those kids.