Nephthys is first mentioned in the pharaohs' funeral hymns, carved inside of their pyramids during the Old Kingdom. Some hymns talk about how nice she is. Others call her names (like an "imitation woman"), and some emphasize how scary she is. If you believe these texts, she's fake and her hair's made out of mummy bandages! Please.
Ramses II (called Ozymandias by the Greeks and Shelley in his famous poem) built two large temples for Seth and Nephthys in Sepermeru, near the Faiyum in middle Egypt. Today it's their only home still standing. Most of their houses were wrecked after the priests on Team Osiris started hacking up Seth's images all over Egypt.
When the Ptolemies and the Romans took over Egypt, nobody forgot Isis, Nephthys, and their family. People still painted them on tombs and temples and stelae (like modern gravestones). On the stela of a woman named Nehmeset-Rattawi, Nephthys is fourth from the left, sitting in front of Anubis, Hathor, and Horus the Elder. Nephthys herself is in the middle, behind Isis and Osiris, in a white wig. Late Period painters often made her outfits the exact opposite of whatever Isis was wearing. That's part of why people today think of Nephthys as Isis's "dark twin."
Seth and Nephthys were the gods of Egypt's desert oases. It's creepy, but probably fitting, that Egypt's biggest cache of mummies ever found comes from one of these oases. Bahariya's Valley of the Golden Mummies, surveyed by famous Egyptologist Dr. Zahi Hawass in 1996, contains up to 10,000 Roman and Egyptian coffins and mummies dating back centuries. That's a whole lot of Halloween bling.
Way back in super-ancient Egypt, the temple of Kom Ombo was dedicated to the brothers Seth and Horus the Elder. When Seth killed Osiris and everybody got mad at him, he was removed from Kom Ombo and replaced his son Sobek. Nephthys didn't get removed. They just removed her trademark hat, changed her name to Tasenetnofret, or "the Good Sister"—a title she uses when helping Isis with funerals—and left her on Sobek's side of the sanctuary. Sneaky.
In Ra's city of Heliopolis (Iunu), Nephthys was associated with the Temple of the Bennu. A bennu, or white heron, is the ancient Egyptian phoenix bird that was a symbol of the rising sun. Later on the bennu and phoenix continued to be her symbols—makes sense for a goddess of death and change—and in Diospolis Parva, where the bennu and Nephthys were BFFs, she got a special temple as the "mistress of the phoenix."
Nephthys helps her sister Isis recover her dead husband and prepare him for burial. Together the sisters are called Ma'ati, or "two truths," and the judgment hall in the afterlife where Osiris judges the dead is named for them. They get to sit on the steps with him and welcome anybody who survives the weighing of the heart. Thankfully, they've gotten over that whole awkward phase when Anubis was born. Thousands of images in temples, tombs, and papyri from the Middle Kingdom into the Roman period show all three hanging out in the hall.
Along with Isis, Nephthys also appears on the coffins and sarcophagi of dead people, mourning for them and welcoming them into the afterlife. Fancy painted coffins from the Middle Kingdom into the Roman period often depict the two sisters either with the soul of a dead person, or with the Djed pillar of Osiris, the hieroglyph for stability and life after death.
In an ancient story in Papyrus Westcar, Nephthys and Isis secretly help some pharaohs to be born. Isis kneels by their mother Reddjedet's feet, and Nephthys stands behind the bed. Why? Childbirth was dangerous in ancient Egypt, and many babies and mothers did not survive. Isis is the goddess of life, so she waits to bring out the new baby. Nephthys is the goddess of death, so she stands behind the mother to take her if she dies. Isis and Nephthys also do this for dead people. In any image where Isis and Nephthys appear with a mummy, or Osiris on an embalmer's table, Isis will be near the feet/bottom/front, and Nephthys will be the head/top/back. If the goddesses are standing side by side, Nephthys is always behind Isis from the viewer's perspective.