The number and genders do not matter. Be nice to your sibling or siblings.
Petty meanness and squabbles are part of family life. One or other or both parties are to blame. This code is a general request to stop it, cool it, lay off, take time out, etc. 'Whoever is to blame, stop it!'

Quite apart from tense moments the code is a general exhortation to be nice to other siblings. It may be that a boy or girl needs help. It may be that a boy or girl is being left out. There are many situations which might benefit from increased 'niceness'. Long-standing sibling jealousy and rivalry are not likely to be affected by this code and need deeper attention. The code can, however, deal with moment-to-moment troubles.