I’m my brothers keeper or helper to the extent he can’t help or support himself, such as genuine disabilities. Beyond this, he needs to take responsibility
You are all ignoring the fact that "keeper" - as used by Cain - meant something akin to "animal-keeper" or (as we would say today) "herder."
One still sees the true meaning in the modern term "zookeeper."
Cain was - rather insolently, to be sure - posing the rhetorical question: "Is my brother an animal? Do you expect me to hold him in a 'keep' him the same way he 'keeps' his sheep?"
Regards,