It is calculated by taking the total number of people minus [brothers_only] people that have brothers only, minus [sisters_only] that have sisters only and minus [both] that have sisters and brothers.
[brothers_only] pertsonek daukate anaiak soilik, [sisters_only] arreba edo aizpak soilik eta [both] pertsonek daukate bai anaia bai arrebak/aizpak.