Brother

The cause of this phenomenon in which sibling warmth is only correlated with risk taking behaviours in brother pairs still is unclear.

[4] Some studies suggest that girls having an older brother delays the onset of menarche by roughly one year.

[7] Females with a twin or very close-in-age brother, sometimes view him as their male alter ego, or what they would have been like, if they had a Y chromosomes.

The relationship of brothers is laid out with the following quote: "The friendship of brothers has the characteristics found in that of comrades and in general between people who are like each other, is as much as they belong more to each other and start with a love for each other from their very birth, and in as much as those born to the same parents and brought up together and similarly educated are more akin in character; and the test of time has been applied most fully and convincingly in their case".

Aristotle also adds this on the matter of retaining the action of doing good once imitated: "Once the habits of ethics or immorality become entrenched, they are difficult to break.