How to improve siblings relationship

[4Marmalade]

My oldest two dc's are in a bad place right now with their relationship.  They are 8 and 5-1/2.  I know to expect bickering, disagreements, yelling at each other, etc... but it seems excessive right now.  We are working with ds1 (with some outside help) as he has some anxiety issues which don't help the situations.  He has a very strong need for control.  I feel like we already do what we should be doing in terms of letting them work it out and only intervening in cases of aggression or name-calling.  But sometimes, it just feels like they are in this negative spiral and they have forgotten that they are able to get along and have fun.  That things between them can be fair and they do do nice things for each other.  

Are there any things that you do at home to help the relationship along without getting too involved?  Any games that you play?  Like "let's have a be extra nice to whoever day"?  (totally thinking off the top of my head).  Some days are just really bad and so discouraging.  I'm hoping as we work with ds1 things improve but we need to so something right now.

[K1329]

I found the book "Siblings without rivalry" to have some good tips in it. I also frequently remind my kids about the importance of family - and that getting along is part of being a family - and that we simply can't tolerate meanness, etc. to a family member. But, the number one technique dh and I use is separation. If the kids (4 & 7) start getting into that downward spiral, I'll take one to the store, arrange a playdate for one, do something so they spend some time apart. Usually, after a couple of hours apart, they're fine and can play together again without serious conflict.