Fermuba is a daughter of Goshnar, the greatest necromant. She is a sorceress and currently hunted down, so the Brotherhood of Bones has had agents deployed in the city of Carlin that take care of her. The similarity in names and the comment from The Bone Master that Ferumbras is Goshnar's grandson makes it very probable that she is Ferumbras' mother. Xed tells us that Hugo is rumoured to be Ferumbras' father, so a Hugo should be Fermuba's husband. If this rumour is true it's still open to debate which "Hugo" he means, as there is a total of 5 mentioned around Tibia.