The only reason not to translate a Japanese term, is to emphasize a cultural difference. If a girl calls a man "Oniichan" instead of "Bro," that tells me she's is either a native Japanese speaker- in which case, the story should be of her experiences as a "fish out of water"- or she's an obsessive anime otaku.
Leaving a term untranslated, is a sign of either laziness, apathy towards potential customers who aren't yet as into anime and manga- VERY BAD for business- or contempt towards those who aren't as big an otaku as the translator- again, VERY BAD for business.