Exactly. That’s another important angle. The younger brother will start asking questions at some point, too:

“Why am I treated differently?”
“Why did you do all that to my brother?”
“Were you warned against doing this?”
“Is it ethical?”
“Is it logical?”
“Is it fair?”

If the parents don’t provide a logical explanation (and they won’t) – that could be a mitigating factor – this won’t end well for them (only them or all of us, depending on the core values of the brothers).