Attention-deficit/hyperactivity disorder (ADHD) is among the most common mental disorders in children and adolescents. While most studies in this field have focused on the genetic and neurobiological underpinnings of the disorder, research focusing on the family environment as a critical context contributing toward the manifestation and maintenance of child ADHD symptoms is still less extensive. Therefore, the aim of this study was to examine longitudinal and bidirectional associations between child hyperactivity, mother-child negativity, and sibling dyad negativity. Data were analyzed of up to 4,429 children from the Avon Longitudinal Study of Parents and Children, a nationally representative prospective birth cohort study in the United Kingdom, at three time points (T1-T3, children aged 4, 7, and 8 years). At baseline (T1, n = 4,063), the child sample (98.8% White ethnic background) composed of 51.6% males. Child hyperactivity symptoms, mother-child negativity, and sibling dyad negativity were examined based on maternal reports. The random intercepts cross-lagged panel model was adopted to parcel out between-family differences from within-family fluctuations and test bidirectional associations. On the between-family level, families with higher child hyperactivity reported higher mother-child and sibling dyad negativity. On the within-family level, unidirectional spillover processes between sibling dyad negativity and mother-child negativity as well as between mother-child negativity and child hyperactivity emerged. Future work in the area of child hyperactivity should implement a transactional family systems approach incorporating both parent-child and sibling subsystems. Interventions to reduce negative interactions between parents and hyperactive children may offer promising improvements in child symptoms and thus alleviate family burden. (PsycInfo Database Record (c) 2024 APA, all rights reserved).