What is Orlha's relation to Tia, and vice-versa?
Orlha and Tia are twin sisters, separated soon after birth because of a superstition. After the split of dimensions, the Another World version of Orlha felt a connection to the Home World Tia, who had fallen ill. Orlha eventually crossed the dimensions with Serge and reconnected with Tia. Tia's Another World counterpart and Orlha's Home World doppelganger are unaccounted for; it's probably that they're both no longer in El Nido, or perhaps dead. Nonetheless, Tia's speech during their reunion is a little mystical, giving rise to a theory that they're equal dimensional equivalents (meaning there are no other copies of Tia or Orlha).
This theory operates on the assumption that Tia and Orlha's "meeting" was truly for the first time. This is probably precluded by in-game events, but the events surrounding the two are mysterious (the dialogue is even inconsistent at one point, as Orlha claims to be Tia's "big" sister, despite the fact that they're same-age twins). Here goes:
Tia and Orlha are a special case; they appear to be dimensional equivalents of each other, but at the same time exist as independent people. For instance, instead of, say, the dimensions splitting Van into one who paints and the other who looks for cash, the dimensions might split one person into the normal Van and another into Cliff, a completely different person. This is possibly the case with Tia and Orlha, who knew of each other but at the same time "never met" until Orlha crossed the dimensions with Serge and visited her "sister." This is contrast by statements by Orlha that she remembers her sister in early childhood, along with her mother and father. Since the two share this special status, it is unknown how they would exist in the unified dimension (since only one version of the "Orlha / Tia" personage could continue existing). Since Tia does die in Home World, Orlha has a better chance of being the true version of herself and living in the Ideal Timeline, with Tia perhaps an anomaly resulting from an imperfect dimensional split. Notwithstanding the conclusion, Orlha will probably at least remember her sister from the adventure.
From: Common Questions