Brothers and sisters are, more often than not, children’s first playmates and adults’ oldest  friends. BUT sibling relationships play out in unpredictable ways with  unpredictable results. Brotherhood and sisterhood can teach social skills and help us learn to resolve conflicts or cause life-long social dysfunction.   New research indicates that, for many brothers and sisters, sibling  relationships yield mixed results. More interestingly, that same  research, which represents an early attempt to sort through so-called Sibling Effects, keeps falling back on one key point: the effects of sibling relationships in childhood echos through the rest of our lives. “Sibling relationships influence children’s adjustment and development about as much as parenting does,” Mark Feinberg, who teaches human development at Penn State University.