Introduction: The age of preschool children is preoperational stage development for the formation foundations of their mental and emotional. In this period, children are sensitive to receiving stimuli in their environment. So that children who have siblings are vulnerable to sibling rivalry, namely unstable emotional control, which can affect instability of learning motivation, resulting in decreased intellectual potential. So the role of parents is needed to prevent the negative impact sibling rivalry. Methods: This study used a cross-sectional approach with a non-probability sampling method. The populationin this study were parent who had preschool in Pembina Kindergarten Kediri with 85 parents and obtained a sample of 30 parents. Selected using purposive sampling techniqe. Sibling rivalry was an independent variable. The knowledge and development level were dependent variable. The data were collced using The sibling rivalry questionnaire and knowledge questionnaire that have been a validity and reliability. The test pra screening development (KPSP). Data analysis used Spearman-rho. Results: The results of the research showed sibling rivalry level have a significant correlation with knowledge (r=0,405, ρ value = 0,027) and development level (r=0,722, ρ value = 0,000). Discussion: Parents have several children, one of whom is of preschool age, has the potential to experience sibling rivalry, it is advisable to pay attention to the knowledge of psychological aspects to stimulate child development so that it remains optimal.