Good points, CR. If TH was abusive, it's possible that he was not reported due to fear of retribution. She may have worried that there wouldn't be enough evidence for anything to be done and that PH, allegedly a victim of abuse herself, wouldn't have been cooperative. We have to keep in mind the events described by SB's aunt took place in the 80s and 90s, when domestic and child abuse was viewed and handled very differently than it is today (and today we are still lacking in how these incidents can be reported and treated). Plus we should remember the fact that SB's aunt was a teenager at the time, and we don't know what the family dynamics were like.
For me, of course I would report something abusive like this no matter what, but not everyone feels that way, so I wouldn't be quick to accuse someone of lying only because they didn't report the abuse sooner. Yes, she waited until after SB was killed, but perhaps because she, as an adult, felt guilt over not speaking up sooner. Yes, she waited until after TH and PH separated, but perhaps because that meant she no longer feared that speaking up would put her sister in danger. None of us know the real reasons, so if we're gong to make assumptions about her motivations, we should be open to all reasonable possibilities. Otherwise we are more likely to arbitrarily include or exclude information so that it fits in to our version of the truth instead allowing information to lead us to the truth we are all seeking: who really murdered those boys?