The objective of this case report is to illustrate the diagnosis and classification of periodontitis according to the 2017 classification system as recommended in the British Society of Periodontology (BSP) implementation plan. We describe two cases in the form of a pair of siblings, who developed periodontitis very early in life. A 19-year-old female was diagnosed with 'generalised periodontitis; stage III/grade C; currently unstable'. Her 14-year-old sister was diagnosed with 'localised periodontitis; stage II, grade C; currently unstable'. The present case report presents an example for the application of the new classification system and illustrates the importance of a periodontal check for children and adolescents and/or their relatives.