When kids fight it's best to work with them separately because each child needs a specific plan for dealing with the other. Furthermore, trying to train them at the same time often results in resistance or even allowing kids to gang up on you, hindering the process.
Here are some other ideas for addressing sibling conflict that you might find helpful.
Parenting Rules for Addressing Sibling Conflict
1) Remain calm.
Don't add your emotion to the existing drama (Most important Rule)
2) Separate kids.
or they feed on each other's explanations.
3) Don't dialogue with a child who is emotionally overwhelmed.
It usually escalates the problem.
4) Require a break to help a child settle down.
Calm the heart before trying to teach it.
5) Empathize with the child's challenge.
(Yes, I understand, your sister can be annoying.)
6) Ask, "What did you do wrong?"
or "What did you do that contributed to the problem?"
7) Define the challenge more clearly.
"So, when your sister was being annoying, you didn't know how best to handle it so you hit her. Is that right?"
8) Validate the child's thinking and identify the mistake.
"I see you were upset and wanted to do something about it. But hitting her wasn't the best choice."
9) Explore other options with a coaching attitude.
"What better response might you try next time?" "You can do the right thing. I believe in you."
10) Affirm and release.
"Okay, let's try again, I think you can make a better choice next time."
Remember, offended kids generally have three choices: Ignore, Confront, or Get Help. Kids need skills that will help them with all three. You're the coach providing solutions not just rebukes.