Similarity of suicide ideation in 1,055 full- and half-adolescent sibling pairs was assessed using Add Health. In each of two waves of data, between 10% and 16% of older and 11% and 13% of younger siblings report suicide ideation, and within families there was some sibling overlap in suicide ideation within and across two waves of data. Logistic regression analyses revealed that sibling warmth predicted older siblings’ suicide ideation, and older siblings’ suicide ideation and sex composition of the sibling dyad predicted younger siblings’ suicide ideation. Suicide ideation was not conditioned by sibling structural and relationship characteristics for older and younger siblings. Using an index of sibling similarity, previous sibling similarity in suicide ideation was predictive of sibling suicide ideation similarity 1 year later. Sibling relationship characteristics were unrelated to sibling similarity index. The importance of considering sibling resemblance in suicide ideation and taking a systemic approach to adolescent suicide are highlighted.