||This study evaluated acquaintanceship effects on self-other agreement between personality ratings of peers versus siblings. Although Funder (1999) has shown that the longer you know someone, the more accurately you can judge his or her personality, this effect does not always hold when comparing raters from different social contexts. Harris (1995) suggested that the home and school contexts differentially affect the expression of personality traits. Although peers at school may enjoy more disclosure and social intimacy with targets than siblings at home, siblings may have a unique perspective on targets that provides them with greater quantity and quality of trait relevant information. One hundred seventy-six college student participants (targets) completed the Revised NEO Personality Inventory (NEO-PI-R; Costa & McCrae, 1992) to assess their personality traits and a modified Personal Acquaintainceship Measure (PAM; Starzyk, Holden, Fabrigar, & MacDonald, 2006) to assess acquaintanceship with a selected peer and their sibling closest in age. Peers and siblings completed a rater version of the NEO Five Factor Inventory (NEO-FFI; Costa & McCrae, 1992) to rate the personality of the target; peer and sibling ratings were obtained for 67 of the 130 targets with eligible siblings. Self-other agreement correlations demonstrated that siblings were accurate personality informants, although peers had slightly higher accuracy for four of the five traits assessed. PAM scores did not moderate agreement for siblings or peers. These findings provide further support for an "acquaintanceship paradox" when comparing trait judgments of college students by peers versus family members.