- We have special duties, independent of communal enactments, to take care of our siblings, precisely because they are siblings.
Notice that (1) creates problems for a number of moral theories. Utilitarians will deny (1). The mere fact that someone is our brother or sister only makes it easier for us to know how to help him or her, which does not make a for a special duty to take care of him or her precisely because he or she is our brother or sister. It is difficult to see how Kantians could justify (1) without relying on something like a dollop of Natural Law (which Kant himself, but not so much contemporary Kantians, is happy with).
Contractarians can accept (1) minus the "independent of communal enactments" proviso. For perhaps it would be irrational for us to reject a communal tradition of a network of duties of special care that is below a certain level of onerousness, and our community's network includes siblinghood. But the contractarian probably could not object if the community instead had a network of duties of special care that put similar emphasis on, say, first-cousins-of-the-same-eye-color instead of siblings: what one gets from the contractarian structure is at most the irrationality of rejecting whatever one's community network of duties of special care is.
Divine command theorists can perhaps accept (1), though one might worry if the "precisely" in (1) is correct if this happens also because of divine command.
Natural law, of the well-developed moral theories, is probably the theory that best passes the test of fitting with (1). So, (1) provides a kind of argument for natural law theory (and to a lesser degree for divine command theory).
Does (1) matter? I think so. It is an important moral insight that somehow all human beings are brothers and sisters, but this moral insight is unhelpful, and maybe harmful, apart from (1).