Holy shit, that's fucking brutal. Well, if I may weigh in...
First of all, "inappropriate contact" is a pretty goddamn vague term. I mean, we can assume that it couldn't have been good if the boy felt guilty enough to confess, but even then it's still kinda vague. Secondly, as I just said, the boy must have been feeling pretty bad to come out and admit it. To go and turn that into physical/mental abuse followed by murder is pretty fucked up. Jesus Fucking Christ!
I can just imagine the conversation with the daughter, too - especially when she grows up and is able to comprehend it a little more. I mean, I can imagine learning that your brother was brutally murdered, even over something like abusing you when you were 3, has got to have a pretty negative effect on the psyche.