Examined family- and sibling-type differences in sibling relationship quality and links between sibling relationship quality and child behavior problems among 192 families with a 5-year-old target child and an older sibling. Found that sibling negativity was highest in single-mother families. Found that full-siblings were more negative than half- and stepsiblings. Found some evidence that sibling antagonism related to child behavior/emotional problems, with effects moderated by family type. (Author/KB)