Why are we are sons of God, not sons of Jesus ? Why are we brothers & sister of Jesus?

I really like your question. It does provoke a little thought even for me as a Trinitarian. My best answer is that Jesus is God as a Son (second Person of the Trinity). Jesus is most accurately referred to as the Son of God. And since we are the adopted sons of God, we are the brothers and sisters of Jesus. But we are also the the sons & daughters of ONE God.