"It's not FAIR" has become the new mantra between you two.
You are constantly comparing the size of each other's cookie, or the amount of time one got to play video games versus the other - or who got what new thing when.
The tough thing is that life just isn't fair.
If it was fair, then only good things would happen to those who make good choices, and bad things to those who make bad choices.
But the reality is, people who make bad choices sometimes live a glorious life.
Here on earth.
We have to remember that we are not accountable nor held in comparison to each other.
Our reward is in heaven.
Thus, life here - isn't fair. It just isn't.