You EXPLAIN child development to them.
I did that with my eldest child, once I had my 2nd child.
THAT worked.
SHE understood, even if she was 3 when I had my 2nd child.
If my daughter could understand that at that young age, an older child can too. What her younger brother was like developmentally and what he can/cannot do, in contrast to her and what she can or cannot do, and that she was like that too, when she was younger.
THEN you teach them what "family" is, and that being a sibling means having each other's back.
That they need to be a 'TEAM.'
Then you also have to let each child, per their age, have their own things, do their own things, and have alone time too... and be taught that they can ASK for that, when needed.
AND teach them about respecting each other's privacy... and that everyone is DIFFERENT.
That, they are NOT NOT NOT, the same.
And that's fine.
They are individuals.
So rejoice in that.
DO NOT LET "bullying" behavior to happen, among siblings.
My sibling was a MEGA BULLY and it was so hated and so OPPRESSIVE, for me, the younger one. It was MISERABLE.
DO NOT ALLOW your eldest, to BULLY.
Period.
CALL her on it.
POINT blank.
THEN, each day, make your kids DO something nice for each other and to say something nice about each other. Like 3 things. AND to do it, for you/Daddy too.
TEACHING them, about thinking OUTSIDE of themselves.
It takes, learning.
My kids are 4 years apart as well.
They get along.
We do not compare them.
We teach them not to compare themselves too.. .that it is NOT A Competition.
They are individuals. NOT the same and don't have to be the same nor do the same things. AND that, they need to watch out for each other... because, they only have each other. And thus, you take care of one another.