This study examined whether the quality of the sibling relationship can alter the negative impact of stress on child well being. Participants were of 311 families (236 two parent families and 75 single parent families) with an adolescent child (M age of child at Time 1 = 11.25, SD = .99, 51% female) who took part in the Flourishing Families Project. Data were assessed using both a multiple time point cross-section and a two wave longitudinal design. Hierarchal linear regression suggested that when assessing the direct effects of sibling on well being, sibling affection is a better predictor of well being, but when assessing indirect effects, sibling conflict is a better predictor. Little evidence was found to support the idea that siblings moderate the impact of stress by buffering or decreasing the negative impact of stress. Results did indicate that sibling conflict was a salient moderator of stress in that conflict exacerbates the already negative impact of stress. Results from this study also suggest that when assessing the buffering or exacerbating impact of siblings, cross-sectional data produces better explanatory power than when these constructs are assessed longitudinally. However a single time point, cross-sectional design does not account for dynamic changes over time in either the sibling relationship, the level of stress or well being. Research designs such as multiple time, point cross-sectional studies or growth curve analyses are recommended.