So DD9 is really angry at DD10 because she is "perfect" and it is spilling over into all areas. I think I am familiar with the type of anger that you are dealing with, unfortunately.
Are things going exceptionally well for DD10 right now? Academically, socially, in sports, or anything else? Really all that matters is that DD9 perceives a (negative) difference between her and DD10. Sibling rivalry can cause a LOT of anger, even when it is something that doesn't seem like a big deal to the parents.
If you can figure out what it is, try to find something, anything - really, that DD9 does better than DD10 and make a HUGE deal out of it. HUGE! Try to find something that she can excel at the DD10 doesn't do. Build up her self confidence.
You may even want to consider taking her to a therapist. Dealing with siblings can be very, very difficult on top of dealing with being different from the other children at school.
