M.L.
I think you're getting a good start. You could suggest that they see what they could do for each other secretly. One child sneaks in and makes up another's bed, for instance; another sneaks out with the trash even though it's a sibling's job.
Generally brothers and sisters really love one another, and even like each other a good part of the time, but that liking gets buried under a neverending avalanche of petty complaints, fatigue, and negativity. Some families try to make it a practice that, when A says something negative or critical to B, A needs to say six positive things to B... not in a strict, legal sort of way, but as an attempt to build one another up whenever they can. It's an constant effort, but worthwhile, not to let negativity or pettiness RULE us.