reason for this is partly historical. In the previous versions of BOADICEA (BWA v3 and BWA v4 Beta), the user could only specify identical twins when building pedigrees online - not triplets or quads. And when we designed the corresponding BWA pedigree data formats, we had to ensure that they were consistent with the data captured with the user interface. The new CanRisk Pedigree Data File Format is partly based on the these earlier BWA pedigree data formats, and so it has inherited this limitation.