The sharing of one allele and the sharing of no alleles at a polymorphic locus of high heterozygosity provide limited information for and against sibship, respectively. The sharing of two alleles produces strong evidence favoring sibship. In a given case, the study of more than three polymorphic loci of high heterozygosity may be needed to develop the evidence that two people are siblings. The general logic and methods used for siblings apply to kinship analyses of other two-person pedigrees.