Hi Meilan,
You bet. B used to come to me with all his complaints. Still does nowadays but less frequent. I'm not exactly sure what I did. But I think treating him as an "adult" helps. Whenever he comes to me with his complaints, instead of telling him what he should do, I got him to analyse the situation and find a solution for himself. Which, of course, will involve a lot of time and patience. I'll listen to his complaints and try to understand how he perceives the situation, get him to talk about his feelings, and how he wants things to be and the type of actions he think he can or should take to alleviate his bad feelings. Most importantly, I always advise him to approach the aggressor to tell him/her his unhappiness and work things out with him/her in an amicable manner (I'm still working hard on this). I'm glad that having a younger sibling helps him a lot in this aspect. Whenever they have conflicts, I am there to assess and direct them to choose the "appropriate" course of action through reasonings. I take neither sides. Both are in the wrong when they start squabbling. For instance, if C snatches a book from B, and B shouts and tries to snatch it back. I'll first reproach C that it is naughty to do so (explain to her why blah blah), then ask B if he has done the right thing, and if he would like it if C does the same thing to him in future, then wrap up the case by walking him through what he could have done. Oh yes, both would have to apologise to each other. Another thing I realise is - my reaction matters a great deal whenever someone does something naughty. That's why I try to be careful of how I handle their misbehaviour too.