The purpose of this study was to assess the influence of sibling relationships on the development of maladjusted behaviour among pupils in public primary schools in Yatta Sub-County, Machakos County, Kenya. The study was guided by the Ecological Systems Theory. A mixed methodology was applied and the concurrent triangulation design in which the researcher implemented the quantitative and qualitative methods during the same timeframe and with equal weight. The target population consisted of 64 headteachers, 128 teacher-counsellors, 512 parents’ representatives and 4352 pupils in classes VI & VII all totalling 5056. Using the Central Limit Theorem, 12 public primary schools (18.8%) of 64 and 200 respondents were sampled. Stratified sampling was used to create four strata based on the number of zones in the Yatta Sub-County. From each zone, three headteachers, three parents’ representatives, 18 teacher-counsellors and 26 pupils in classes VI & VII were selected using simple random sampling to eliminate bias. This procedure enabled the researcher to sample 12 headteachers and 72 teacher-counsellors. Data analysis began by identifying common themes. Qualitative data were analysed thematically along with the objectives and presented in narrative forms. Quantitative data were analysed descriptively using frequencies and percentages and inferentially using Pearson’s Product Moment Correlation Analysis with the help of Statistical Packages for Social Science (SPSS Version 23) and presented using tables. The study established that cases of maladjusted behaviours among pupils are on the rise with sibling relationships and interactions being the main determinant. In other words, activities that siblings engage in such as modelling juniors, playing together, engaging in social interaction activities and punishing juniors contribute to the development of maladjusted behaviours among pupils. Thus, the study recommends that parents should design age-appropriate tasks for older siblings to undertake which may enable them to act as role models for their juniors.