The evolutionary biologist J.B.S. Haldane, when asked whether he would lay down his life for his brother, replied, 'No, but for two brothers or eight cousins.'

Sashin's notes

If all our behaviours are performed for the survival of our genes then the life of two siblings would be just as valuable to us as our own. A brother or sister of yours would have half of your genes and a cousin would have an eighth.

This quote was referenced by Steven Pinker in The Better Angels of Our Nature.

It was also mentioned by Richard Dawkins in this talk.