Well, Ran's sister can always show up and try to get Keiko for herself.
So....I keep seeing reference to Ran having a sibling around here...
Am I the only one who thinks Ran is an only child? She surely acts like one, but that aside...
Initially when I read chapter 10, I thought she had a sister. In rereading, it seemed more like that text was a continuation of her friend's account of why Keiko was a bad egg. After all, why would Ran be so naive of Keiko and her reputation if she had a sister to giver her warnings well in advance?
Added to which, the sister is never mentioned otherwise, she's not in the family photo and didn't show up at the father's grave to pay respects for the 49th day memorial....
Even the layout of the text bubbles that I assume lead people to believe Ran has a sister are different from the norm when it's the characters own narration (like here or here or here etc etc) in that they are double bordered.
Just throwing that out there. If there are any other hints that point to Ran having a sibling, please enlighten.