Where does it say the OP never does anything about it?
just reading between the lines to be fair, could be completely wrong:
I immediately took my child away and acknowledged his pain and told him that what they did to him was wrong, then left with him.
What about the little brothers pain? no mention of a consequence there, maybe the other relative stepping in OP felt there was well over the top and no further action was needed, or maybe she had no intention of correcting it. Who comforted the brother that had been bitten?
My child is incredibly caring towards his little brother, with just some rough moments when he is tired or overstimulated
She mentions here that he is rough with the little brother when he is tired and overstimulated, no mention of what action is normally taken, it seems like saying assaulting the younger child is being excused because he is tired and overstimulated so has no control over his actions.
plus the fact that a family member has suddenly, and seemingly out of the the blue, stepped in in a way they never have before seems to indicate that OP is perhaps not correcting this rough behaviour effectively.
I see a LOT of gentle parenting and I do think it has a place, but when other kids are getting hurt because your parenting method is not an effective enough deterrent it is time to look at it again.