I would say..... IMHO - to monitor the situation, but do not do anything that might cause you more damage or your step-brother any undo emotional stress.... for what happened to you may never happen to him.... one reason being that your brother is this mans natural child.
While I was mistreated by my step-father my other siblings were not - I was the chosen one for this mans anger..... my younger brother, this man child with my mom was given the golden treatment at all times..... for he was their natural child together, where as my 3 sisters and 1 brother were not.
Good Luck..... and YES, do let you step-brother know that you are always there for him.
LoVe,
Rhapsody -