I believe the original question concerned what Darwin would predict in a situation where ethics requires self-sacrifice. There is no single Darwinian answer, but evolution, in some species, has produced self-sacrificing individuals.
Evolution does not predict selfish or unselfish behavior. It is irrelevant to questions of ethics and morality.
C'mon guys. One cannot antropomorphize a theory.
And then there is defining selfish.
If it satisfies the parents to sacrifice their own children to benefit the other family, then that is selfish behavior.
As for cats, momma cat is doing what makes her feel good.
And, of course Darwin is irrelevant here except for predicting the outcomes as opposed to the actions.
But; behavior DOES influence amount of offspring output.