Sibling

In a family, a sibling was an offspring born from the same parent or parents as another offspring. In the case of multiple births, more than one sibling (i.e. twin, triplet, or quadruplet) were born at one time.

Romulus and Remus Sylvest were twin brothers. (TV: The Twin Dilemma)

Martha Jones voiced her belief that the Tenth Doctor was going to reveal that was his "secret brother or something", which the Doctor attributed to her watching "too much TV." (TV: The Sound of Drums)