I really want all the translators out there would just leave Onii-chan or Onee-chan and the likes as they are, Big bro or big sis sounds much less natural, and at this point, everyone that go to this type of websites already knew what onii-chan, or nii-chan or aniki or something else means, so just leave it like that so we can feel the original intends of the authors. Thats my opinion