Maribel’s ability to see and interact with boundaries is weaker than Yukari’s much finer degree of control, but given the tremendous growth of Maribel’s power over the course of the Sealing Club stories, it’s possible that Maribel continued to grow stronger until her power became like Yukari’s. The fact that Yukari existed in a time period before Maribel’s birth can then be explained by Maribel traveling through time and space to early Gensokyo before adopting the name Yukari. Maribel’s ability to time travel is established both by her dream of meeting Fujiwara no Mokou and Akyuu’s discussion of a note that was discovered hundreds of years ago. This memo details Maribel’s exploration of the Bamboo Forest and mentions Renko by name.
This would also explain Yukari’s long absences from Gensokyo and unusual sleep habits. Just as Maribel went to Gensokyo in her dreams, it’s possible that Yukari sees the Outside World in hers. InGhostly Field Club, Maribel peers into the Netherworld by tampering with a graveyard boundary, finding cherry blossoms despite the fact that it was autumn. At one point in the games, Yukari muses that she gets to see the cherry blossoms three times because she can traverse the boundaries between Gensokyo, the Netherworld, and the Outside World.
If indeed Maribel transported herself to a period in Gensokyo’s past, then it’s unclear what happened to Renko. Yukari herself has never spoken of the matter, but the simple fact that Renko is a normal human means that she either opted to remain in the modern world or came with Maribel but died at the end of her natural lifespan. Some fans speculate that the line of Hakurei shrine maidens in Gensokyo are descended from Renko, citing the similar appearance between her and Reimu. However, there is no concrete evidence to support this idea, leaving Renko’s fate very much a mystery.