How could this POSSIBLY be child abuse?

It's INCREDIBLY difficult to actually PROVE date rape. For him to have actually been convicted, there must have been a TON of evidence working against him-DNA, bruises, etc. The majority of cases of date rape are never reported/dismissed because there is typically a lot of gray area and a lack of evidence. If he is actually registered, he is likely not telling you and your sister even close to the whole story.