If you are talking of the Dreth in Riften and if he is related to this Dreth, I think they are related, because the Dreth in Riften (I beleive his first name is Romylen or something) often ma say, "A Dreth never forgets a traitor!" Normally, someone would not speak this unless they are part of a large family.
If you do indeed search up "Dreth" on the Wiki, there's at least five members with the same surname. Though I don't think they actually ever mention themselves, doing a bit of reasearch, I found out that Madsu Dreth lived on Vvanderfell; and Valen Dreth , the one who makes fun of the prisoneer at the start of Elder Scrolls IV: Oblivion, notes to the Dunmer Male the following words:
"'Hey, there! You! Kinsman! I haven't seen another Dunmer in here in I don't know how long. Where you from, huh? Vvardenfell? You got a wife back home? Tell you what. I'm getting out of here in a couple of weeks. When I get back to Morrowind, I'll look her up. She must be so lonely. Don't you worry, eh? I'll take care of her long after you're dead. Oh, that's right. You're going to die in here!"
Thus possibly hinting that he previously lived in Vvanderfell before being imprisoned, which leads to the conclusion that he may be related to Madsu Dreth from Elder Scrolls III: Morrowind. Also, as annoying some may find Romlyn, I think he's not just doing a random lie about his ancestry in Valen.
Madsu --> Valen --> Romlyn. And there's also Hloval from Elder Scrolls IV: Shievering Isles D.L.C., but I don't think he mentions anything about his family. Oh, and Taron from the Elder Scrolls V: Dawnguard D.L.C. .
I just walked in to the Bee and Bard and heard Romlyn talking about how one of his ancestors Valeyn escaped the imperial prison killing six gaurds, Kerrva ( the bar maid ) then sarcasticly replies “was this before or after the mythic Dawn killed the empire and he fought them off single handidly” suggesting that Romlyn is related to Vaylen and they believe he was the main protaginist in the game oblivion.
yeah, that could be true, once asked someone if they knew a noah smith, they replied "can you be any more generic?", so its possible that dreth is very common, or that they are related, but this matter intruiges me.
me again, now that i think about it, they are related, maybe not closley, but eventuly in their bloodlines, all the dunmer would have to share at least one ancestor.
Madsu and Valen are most likely brothers. They both share similar features, that being white hair, greenish skin, and a low propensity for growing facial hair. Romlyn could be either Madsu's or Valen's son due to also sharing the white hair, green skin, clean-shaven phenotype. On the other hand, Taron shares none of these features (blue skin, black hair, beard and stache) and appears to be much more wealthy than the other three. He does dress more in a Dunmeri fashion than someone of Skyrim so it can be assumed that he's somewhat related though not closely, but more as a distant cousin of some sort.