Teach them what "family" is.
Teach them that they are 'siblings'... and that means looking out for each other and having each other's back.
Everyday, tell them to try and do something for the other sibling or you.
Practice.
Tell them, to think of how they can 'help' each other or to think of something they like about the other sibling and/or you and Daddy.
Practice.
If they can write, have them write it down too.
Keep a checklist, so that when they do do it, it is 'visual' and they can see their 'accomplishments.'
Sibling rivalry, happens.
But teach them to COOPERATE.
Not everything is a 'competition'
Reinforce the positives they do. Not the fighting.
If they fight over something, then take it away. From BOTH of them.
Teach them, what family is. And the 'role' of what a sibling is.
Don't "expect" them to HAVE TO share everything.
Teach them how to TAKE TURNS.
I don't make my kids share everything. Some toys, are just too special for one child or the other and they don't want to 'have to' share. That's fine.
You teach them to RESPECT the other's belongings.
That is a lesson too.
all the best,
Susan