Siblings rivalry

It’s very normal. Speaking as a middle child. Most of the time is because she felt the unfair treatment (comparisons etc etc) towards the other siblings. You may not realize it, but you may have done it subconsciously. Like my mom, she always did that to me. It’s like she hates me or something lol! Whereas when my older and younger siblings, regardless of what they did, it’s always “negotiable or understandable”. You can either speak to your 9 year old to understand why she is doing that or just ignore it. I used to fight with my siblings but after a certain age we became close. No more fightings/arguments. We treat each other like a young adult. Try to include some 1 to 1 activities with your 9 year old, then slowly talk to her. Ask her how or what she thinks. They will open up when they feel comfortable to talk to you. Good luck!