User blog comment:B214/Question and Answers 6/@comment-71.15.106.32-20170413060440/@comment-29963364-20170413062208

I'm thinking greate-greate grandfather (at least).

I base this on what we've heard of the Bael Clan from Magdaran Bael in volume 21.

The current Lord Bael (Sairaorg's and Magdaran's father) is rather incompatent as the Head of the Family, and is always going to the "previous" Head (his father), and Zekram for advice.

Since only the "previous" Head and Zekram mentioned, we can assume that Zekram is Sairaorg & Magdaran's great-grandfather (if Magdaran didn't leave anyone else out).

Also, Mephisto Pheles said that he new Rias' grandparents, greate-grandparents, and greate-greate-grandparents. He was likely refering to the Gremory side of Rias' family.