Hi TK,
Yes, just adding in my .02 to these great responses.
Boredom and Anxiety are huge issues with my SS10. The interventions and meds have helped reduce it by 50% but my SS10 will still cry if he feels bored sometimes.
My suggestion is to have all 3 kids come up with lists of things they can do when they are bored.
Work with all 3 on chore charts with rotating chores that are all the same.
My SS10 will sometimes pull a cinderella attitude ( he has 2 Step sisters as well

) and having these chores decided on ahead of time help.
Develop some simple behavior rules with them.
Look into a reward system for all 3 kids. It is good that all 3 are close in age so you can get all 3 of them working on this.
Have ALL this (boredom reminders, chore charts, rules) posted on the refrigerator or public place in the house.Also suggest TV chart for who controls the TV remote when. It's amazing how much power our TV chart has in our house!
During summer have those girls in camp. Have as much structure and planned activities and schedule as possible.For these unpacking days-- send all 3 girls out to a movie with a sitter and keep them out as much as possible.
These problems don't go away but the more you anticipate the better.
Allow SD9 to go to her room or find some other soothing activity.
One example for me of a particular anxiety: My 3SS all have some types of food (when/what/why are we eating!) anxiety issues.
So, now, I make sure I know what we are having for dinner ahead of time so I can tell them when they ask. I also will go home and cook dinner first then pick them up from school so there is less empty time waiting for food.
On the weekends, Dad takes the boys out for a snack or lunch. They love this. Food is huge with many boys.
For girls it may be something else- like anxiety over what they are going to wear or homework assignments and having fits about it right before school. I would have them plan the night before and set aside time for that.
I don't know exactly what is going on day to day in your house that causes her anxiety but planning ahead really really helps.
mamachelle