Persistent thumb sucking beyond early childhood may be associated with dental, dermatologic, and psychosocial complications and can prove resistant to conventional behavioral interventions. This case report describes the use of clinical hypnosis for persistent thumb sucking in nearly 6-year-old monozygotic twin girls. During the first session, Twin A and Twin B participated simultaneously in a developmentally adapted hypnotic intervention. At the second session, Twin A declined further hypnosis and engaged instead in a supportive therapeutic conversation, while Twin B received an additional hypnotic intervention. Following the completion of the intervention, the mother reported an almost complete cessation of thumb sucking in both children, with the improvement largely maintained at the 1-year follow-up. Beyond symptom reduction, the mother also reported increased differentiation between the twins and more individualized relational patterns within the family. Although outcomes were based on maternal report, this case highlights the potential clinical value of brief pediatric hypnosis and the possible relevance of twin and sibling dynamics in the treatment of shared habitual behaviors.