My brother's/sister's keeper?

Question

I've heard it several times, now, even as the title of a book.

So what does it mean to be your brother's/sister's keeper?

Answer 1

It might mean someone who looks out for their brother/sister

Answer 2

It means that you are there for them whatever their wrongs. It goes further than that  as it also means you are responsible for them as they are for you. It's an expression of the ultimate bond in that you are prepared to take their place in the good and bad in life - and they for you. Very few are actually capable of meaning this in it's truest sense. Would you die so that your brother may live...............?