User blog comment:Crimson and Darkness/DxD Updates 2~/@comment-39089654-20190616194100/@comment-2208155-20190619111508

Philidor98 I think that.. All the 72 pillars allowed their sons/daughter marry the others 72 pillars descendants during generations and all of them are like one big family...

Rias mother is a bael married with one Gremory.. Maybe someone from Sitri family married one Gremory or Bael time ago and for this they are family..

I think that Rias called Lady Phenex uncle too..