Sotha Sil belonged to a minor house, Sotha. Almalexia would have nominally belonged to Indoril, considering she was married to Nerevar. Vehk? We don't know. It might have been a non-issue, as he was born of quite humble stock from a small village who might not have belonged to a great house (at the time, the divide between Ashlander and Houseman might not have been so great).
Indoril are a pious house strongly affiliated with the temple; to the point where the armour belonging to ordinators is called by some "Indoril armour". When Morrowind surrendered to the empire, many nobles of took their own lives than see Imperial boot on holy Veloth.
The Dres are are slavers skilled at taking captives from the borders of Black March and Elsweyr. These slaves are put to work on their sizeable plantations. A certain book suggests their upper echelons are vampires, but this is crap. It might also be propaganda from Helseth's camp.
Vivec has this to say about the Dres: "House Dres represents the past of pre-Tribunal Great House culture, a persistent tradition of Daedra- and ancestor-worshipping civilized Dunmer clans."