"Diamond sister": The sister(s) in your sorority that are paired together to have a mentoring/guiding role in the chapter. So I have a diamond sister that mentored me, and then I was a diamond sister/mentor to another girl. Put more simply, it's the "big/little" or "mom/daughter" relationship of our sorority.
Get it? Did that bore you? Too dang bad.
Okay, so the Diamond Sister I mentored is Emily. I adore her. I fondly called her my red-headed step-child for a brief period of time. And I miss her like crazy, even though I don't tell her nearly often enough.