Wow - my kids (DD10 and DS7) do this sometimes, but not as bad as it sounds at your house. My DD is also 'high needs' with a lot of anxiety issues that take up a lot of parent time. DS often tries to insert himself in the middle when I'm dealing with her - boy it's hard!

He also tries to get DD's attention - a lot! He'll go and just stand in her room and of course then she'll yell at him to get out - goes downhill from there.

I don't have any good advice, but there are a couple of books I've been meaning to take a look at:

Peaceful Parent, Happy Siblings: How to Stop the Fighting and Raise Friends for Life http://www.amazon.com/gp/product/03...=yourparesolu-20&linkId=4YCOWSJBTYMZPFB6

I really like Dr. Laura Markham - I find her parenting advice very gentle and practical. Her web site is http://www.ahaparenting.com/

Another one recommended to me is in the American Girl book series:
Siblings: You're Stuck with Each Other
http://www.amazon.com/Siblings-Your...4_53?ie=UTF8&refRID=0AMHXH2WKX5RWKH2Y7S6

Again, I haven't read it yet, but a friend of mine said her DD found it really useful.