The present work describes the assessment process, evaluation strategies, and cognitive intervention on a 9 years old child with selective mutism (SM), a monovular twin of a child also affected by mutism. Currently, the cognitive behavioral multimodal treatment seems the most effective therapeutic approach for children diagnosed with selective mutism (Capobianco & Cerniglia, 2018). The illustrated case confirms the role of biological factors involved in mutacic disorder but also highlights the importance of environmental influences in the maintenance of the disorder with respect to relational and contextual dynamics (e.g. complicity between sisters, family relationships). The article discusses furthermore the importance of an early diagnosis as a predictor of positive treatment outcomes.