The sources call Urd Neri's nipt (kiswoman), and we can identify Neri as Mimir [Rydberg's arguments]. So Urd is definately related to Mimir, if not his sister. She could be a sister, a daughter or a niece (sister's daughter) according to the uses of the word nipt, elsewhere.
So the relationship is uncertian within that range. Thus, there is certianly room for argument here.
We also know that Night is Njorvi's daughter, and that Njorvi is another name for Mimir [VR again], and we are told that Gudmund of Glasirvellir [Mimir] has 12 daughters. We also see King Nidhad (Mimir in Volundarkvida, has a wife and a daughter named Bodvild.]
The origin of Urd and her exact realtionship to Mimir is one of those gray areas in the myth. I tend to think that Urd is Bestla, primarily because Urd owns one of the 3 wells, as does Mimir, and we find the hrim-thusar in Niflheim with Hvergelmir. This corrresponds to the 3 children of Ymir-Aurgelmir (Vafthrudnismal 32). We find a boy and girl born under his arm, and a three-headed thurs born under his feet. In Havamal 140, we find Odin receiving runes from Bestla's brother (Odin's mother's brother, a sacred realatioinship per Tacitus in Germania ch. 20). And in Grimnismal 29,30 all the gods (presumably led by Odin) gather near Urd's well to hold court, where the judge the dead daily. We also find the youngest Norn Skuld as leader of the valkyries, under Odin's command [Voluspa R20 and R29].
So unless there is an extra generation between Odin's mother and Urd, it appears that Urd is Bestla. However, we do not know what happened to Bestla or Borr, Odin's father, and their names seem to link them to the Tree. So its very possible we have a lost myth where the two of them somehow became the Tree.
We see man and woman made from trees, so this isn't as far-fetched as it may sound. But we have no smoking guns or direct statements to solve the mystery. Thus it must remain unresolved with possibilities. I will add that Eysteinn Bjornsson was against the idea and just did not feel that Urd could be Odin's mother. That left me against the idea for many years, but upon reflection, I think its the simpilest solution without adding an additional generation (i.e. daughters of Bestla and Mimir, becoming the Norns, and one of them --Urd-- owning the well).
We also have the cryptic passage in Vafthrudnismal 48-49, which speaks about Mogthrasir's maiden's, who "glide over the sea", which most commentators think must be the Norns but cannot explain the passage. Finding the meaning of these two verses may be a clue to the puzzle above. I see variotions in the published rtanslations, and there was an emendment to the passage suggested by Gering to "better" understand the meaning, which some of the English translators have accepted, so there is definitely opportunity here to look into this further.
Having 3 norns is from Voluspa 20, however Verdandi is not mentioned elsewhere. Both Urd and Skuld were.
The 3 names are in past, present and future tense forms, so scholars have tried to say Urd presents the past, Verdandi represents the present and Skuld the future, but that is literally an argument based on grammatical forms of the names. Clearly its there in the poem, but may or may not be more than a poetic device.
-William P Reaves, 2021