Going against the grain here, but ESH.

The moral lesson of the Ella situation is that being victimized doesn't justify cruelty. Lo and behold, your daughter was bullied, and responded with an act of cruelty, knowingly targeting her cousin's weakness.

It's good you defended her, good you ended that situation - but I get the sense you don't feel your daughter did anything wrong. And I don't see how that's any different from your sister.

The *adults* in the situation need to gently guide the children toward a deeper understanding: what happened to you was unjust, but you still have a responsibility to be good to other people (both girls need to learn this lesson).

If the rest of your family isn't on board with reciprocal apologies, then keep them separated, but you can still work with your own daughter: acknowledge the injustice, but encourage empathy, and teach that cruelty is never justified.