Saying just that his sister died is slightly misleading given that she was murdered. I see the defence claimed that he blamed himself for her death and this twisted her thinking.
This sentence is clearly outside the sentencing guidelines. Assuming the conviction is for rape of a child under 13 as per the report the minimum sentence is 6 years. Given the description in the article this may be a Category 2 offence for which the minimum sentence is 8 years. The mention of trust in the article makes me wonder if the offence involved abuse of trust. If so the minimum sentence would be even higher - 11 years.
Judges can step outside the sentencing guidelines in exceptional cases. Without having heard all the evidence it is impossible to be certain but it does not sound like the circumstances surrounding the murder of the accused's sister are anywhere near enough to allow the judge to step outside the guidelines, let alone this far. It may be, of course, that the news reports are missing some vital piece of information. If they are not I would hope that the prosecution appeals against this sentence.