Reward. At the end of each school day, students hand in their cards. The teacher counts the tootles, only counting the pro-social behaviors. The next day, the teacher announces the number of tootles recorded and reads some examples aloud and offers praise. The teacher then records the number on a feedback chart. After a certain number of cumulative tootles (set by the teacher), the class is rewarded in some way, either through an activity that the whole class enjoys, a few more minutes at recess, or a special snack.
Repeat. After each reward, the process begins again, perhaps with new, more stringent criteria for tootling and with a different reward.
As Nechama experienced, tootling can work at home too, but is implemented in a slightly less systematic manner. Clearly, your children will not be using index cards to record their siblings positive behaviors. Instead they verbally report the pro-social behaviors throughout the day. The parent keeps track of those tootles and tallies those tootles in a chart. Just as in a classroom, the family members can decide together what kind of reward comes with a certain amount of tootles.
After a few months, tootling becomes instinctive and part of the everyday experience at home. At that point, the tootling rewards itself through a positive environment and better sibling interaction. In that sense, mitzvah goreret mitzvah.