Data from a survey of 200 young adults assessed whether the early nonshared environment, specifically parental differential treatment, was associated with romantic relationship distress through its effects on sibling jealousy, attachment styles, and self-esteem. Individuals who received equal affection from their parents in comparison to their sibling reported equal jealousy between themselves and their sibling, had higher self-esteem, more secure attachment styles, and less romantic relationship distress. Receiving differential parental affection, regardless of whether the participant or their sibling was favored, was associated with more negative models of self and others, which in turn were associated with greater romantic relationship distress. Results indicate that early within-family experiences may be particularly relevant for later healthy romantic relationship functioning.