MissyM, my husband had three, unrelated abusers in his life, and one situation was like your sons'. His primary abuser was not, however, an older step-brother, but an older brother by 7 years. The most difficult thing for him has been the guilt he feels for the acts he was forced to commit with his younger brother. The feeling that his younger brother's abuse was his fault because he was complicit (even though he had endured abuse himself since age 6), and because he did not have the courage to tell to save his younger brother from the same torment. Your older son likely carries this double whammy of guilt too. Shame for his own abuse and guilt for his younger brother's. I hope your oldest son seeks therapy one day for this, because I know he can heal. It can get better as it has for my H since he decided to begin recovery.
My very best in healing to you all.
I am not your rolling wheels, I am the highway
I am not your carpet ride, I am the sky