User blog comment:B214/High School DxD EX/@comment-1359322-20150630104729/@comment-1359322-20150630114614

Then let me ask you this then, does Rias call her mum Venelana kaa-sama? Or does Akeno call her mum Shuri kaa-sama? No they just straight call them okaa-sama, so yes they might be calling their siblings' mum as mothers too.