We've always treated our two as "a team", that's how we described them, and how we set expectations of them working together. We rewarded kind and thoughful behaviour. However, when the oldest was 6 or 7, she started to get quite domineering and a bit mean, so we decided we had to go a bit further to reinforce the team/family idea. We introduced a system called Sister Points. Our daughters were rewarded with points for working together, being kind, achieving things, and deducted points for unfriendly, unkind, or mean behvaiour. They shared the points total, and when they reached certain levels (25, 50, 100, 250, 500) they levelled up as sisters (level 2 sisters!) and we had a family treat of increasing awesomeness. They love games, so they loved the idea of levelling up, and we were rewarded as a family for doing well. It's worked really well - they are very much still a team (they're now 10 and 7) to the point that we don't really need the points anymore!