We have something similar in our house. I work to make sure that our middle child doesn't feel "behind" her sister and explain that she's doing just great for her age and doing much more than I was at her age. I couldn't ______ by the time I was ____. She often gets a hoot out of that.
Our middle child is very much "Mom's helper" and often is very eager to please. I make sure I commend her for her help.
As for the leader older child, we remind that one not to interrupt or to try to "trump" the younger one. So if the middle child says, "I got a sticker on my school paper." The older one often says, "Mom! I got 100% on my spelling test." My DH and I remind the older one that we are talking about the middle child right now, and that she needs to react appropriately to what her sister just said, like "good job" or "that's great". And when the middle one is done talking about her day, we'd all be very happy to listen to how someone else's day went.
We let our kids earn special mommy or daddy alone time by doing some simple chores. More often than not, the middle child gets things done regularly and has earned time away. Usually it's just a trip to the grocery store, but she gets to pick out an extra cereal or snack or a run to Home Depot with Dad where she gets popcorn or a sucker.
Also, we let our middle child select some new clothes at the beginning of the school year and remind the grandparents at Christmas that she could use a new _______. Hand me downs are not as fun as new clothes. So we do our best to let HER pick a few new items or replacements. Often this gets a negative reaction from the older child, who is quickly reminded that she gets everything new.
Finally, we expect each child to "mind themselves" that means no "mothering" each other. This has helped improved tattling on each other, putting hands on each other, etc. When each child simply takes care of themselves it makes life much easier for everyone. Try to ensure that you don't expect your 9 yr old to "watch the little ones"...we all do it. But maybe just the littlest one, if she needs help. The only times our kids are allowed to tell on each other is if someone is going to get hurt or something broken due to naughty behavior.
I'm with you mama. It's not easy - esp when you have to deal with it 24/7!
Happy Holidays