Sibling

A sibling was an offspring born from the same parent or parents as another offspring.

The Thirteenth Doctor recalled having once had sisters. (TV: Arachnids in the UK)

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

Blon Fel-Fotch Passameer-Day Slitheen refered to Jocrassa Fel-Fotch Passameer-Day Slitheen and another as her brothers. (TV: World War Three) Following their deaths, Blon recalled that she had "wonderful" and "bold" brothers. (TV: Boom Town) Glune Fex Fize Sharlaveer-Slam Slitheen and Kist Magg Thek Lutiven-Day Slitheen were brothers. (TV: Revenge of the Slitheen)

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)

The Doctor's TARDIS considered other, fallen TARDISes to be her sisters. (TV: The Doctor's Wife)