Coz she abandoned her in the first place, and she doesn't even apologize for that!! And now she made her cry...
Remember that on some level she was still telling herself that her little sister actually had a boyfriend (as ridiculous as that was) that she was getting in the way of with her desires. So she thought she was making a sacrifice to make her sister happy.
Humu, I guess it's one way to see this...
Particularly, my thoughts are basically the same of Nez's, besides, I think it's a bit early to make conclusions, since the guess that this last chapter would be the Onee-chan's point of view of the past didn't happen, so, even if it turns out not to be the best reason, I don't think Mochi would just make a character plain insensible.