I do know who might have molested his brother. Now, please don't get offended if you are a devout Catholic, but his parents are devout Catholics and raised their boys to be. Both my fiance and his brother served time as altar boys.
Given all the stuff about altar boys and priests in the news in recent years, it is possible that was one possible venue for abuse.
Other than that, his family is small (just the 4 of them) and all the other relatives either live in other states or other countries (in other words, not much chance of an abusive uncle or whathaveyou).