She's not leaving the country for her safety. She's leaving because it's a brand new start at life. In games like these, the client goes off to another country for reasons like those. As for the sister's death, we don't know the true culprit, only that they have a relation to Kusora, the final boss. Meaning it's Part 1 of the mystery. Part 2 will be in a future quest, maybe Kusora's directly. Seeing as the chapter ended with Kusora about to confront the culprit, no they are not brought to justice. We might get their identity in the next chapter.