A key part of the kin selection theory of altruism is, well, that people can differentiate between "kin" and "not kin". A new paper gives a model for how kinship is calculated:

As predicted, the kin detection system uses two distinct, ancestrally valid cues to compute relatedness: the familiar other's perinatal association with the individual's biological mother, and duration of sibling coresidence.

That is, the kinship detector goes off if someone spends a lot of time with your mom, or if you've lived together for a while. The methods used to determine the level of "kinship" you feel for your sibling are the perfect Valentine's day reading material:

Instrument 3: disgust imagining sexual acts with a sibling (Likert)

Subjects were asked how disgusting they would find engaging in various sexual and nonsexual behaviours on a 7-point Likert-like scale (0, not disgusting at all; 6, extremely disgusting). Among these were sexual acts with particular opposite sex siblings. For each opposite sex sibling, independent ratings for passionately kissing, and having sex with 'your sibling' were summed to produce a dependent variable, sexual disgust (Likert).
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Instrument 4: disgust imagining sexual acts with a sibling (rank)

A subset of participants who completed Instrument 3 also completed Instrument 4 (N = 375), which asked participants to assign a unique rank of disgust from 1 (not disgusting at all) to 50 (extremely disgusting) to eight acts, some of which involved sexual contact with a family member, short of intercourse. Using the rank of the sexual act involving a sibling, a variable, sexual disgust (rank), was constructed (women, mean = 47.36, s.d. = 3.99; men, mean = 45.51, s.d. = 9.91).