Indiscriminate altruism: unduly nice parents and siblings

Many animals can identify their relatives and bias altruistic behaviour in their favour. However, recent studies have also uncovered cases where nepotism might be expected but is weak or absent within social groups. For instance, in some bird and mammal species, males apparently feed offsping that have been sired by other males at the same rate as their own offspring. Similarly, social insect workers fail to favour more closely related individuals within their colony. Why is this so?