Short answers, pulled from the story.
A sibling is a relative who shares at least one parent with another person. The word was reintroduced in English in 1903 in an article in Biometrika as a translation of the German Geschwister, having fallen out of use since Middle English around 1425.
Full siblings are 50% related by consanguinity, sharing both biological parents. Half-siblings are 25% related, sharing only one parent. In practice, the actual DNA shared between full siblings averages 50.28% with a standard deviation of 3.68%, due to the mechanics of chromosomal crossover during meiosis.
Three-quarter siblings share one parent while their two unshared parents are first-degree relatives to each other. The children of Charles Lindbergh and the two German sisters Brigitte and Marietta Hesshaimer are a documented example; those children are both half-siblings and first cousins to one another.
Scientific literature reviews that control for confounding variables find minimal effects of birth order on personality. Researcher Judith Rich Harris concluded that any birth order effects may exist within the family context but do not persist as lasting personality traits. Birth order has no genetic basis.
Sibling rivalry is competition or animosity between brothers and sisters. It typically begins at or before the arrival of a second child. Children show sensitivity to differences in parental treatment as early as age one, and by age three they can compare themselves directly against a sibling. Research finds the highest reported competition in the age group of 10 to 15.
The Westermarck Effect is the finding by anthropologist Edvard Westermarck that children raised together in early life are desensitized to sexual attraction toward one another later on. It applies in biological families, adoptive families, and among unrelated children raised in close contact, such as in the Israeli kibbutz system and the Chinese shim-pua marriage tradition.