I mean I understand where you're coming from by saying that it likely very hard for her to understand she was a bully because of what happened to her.

But my main point is that OP doesn't owe her sister sympathy or empathy in this situation. The sister confessed she did horrible things to another person and only Now feels bad about it. But not bad enough about it to not use it to get comfort from her sister.

Maybe OP was too rude with her comment but you can't say that people opening up about making Other People's lives Hell for an extended period of their childhood because they were sad deserves coddling and sensitivity just because they're realising pretty late that their actions were wrong. Apology or not bullies need to be held to a degree of accountability and not everyone has to be kind to them