Isis — Egyptian Aset, meaning "throne" — is the daughter of Geb (earth) and Nut (sky), sister and wife of Osiris, sister of Set and Nephthys, and mother of Horus. Her name's hieroglyph is the throne — the symbol of royal power — suggesting that from her earliest origins Isis was understood as the divine embodiment of the royal seat, the power that legitimises kingship. The pharaoh sits on Isis; she is his throne.
In the earliest texts, Isis appears as a relatively secondary figure in the divine family — it is in the development of the Osiris myth, and particularly in the Pyramid Texts (the oldest religious corpus in the world, inscribed in royal tombs from around 2400 BCE), that her power becomes fully apparent. In the Pyramid Texts, Isis is already the supreme mourner, the searcher, the magician who restores the dead — the one whose lamentation brings Osiris back to life and whose magic protects the dead pharaoh in his journey through the underworld.
What makes Isis theologically extraordinary is her combination of attributes that Egyptian theology usually distributed among multiple deities: she is simultaneously mother (of Horus), wife (of Osiris), sister (of Set and Nephthys), magician (the greatest in existence), healer, mourner, searcher, protector and initiator. She is not a specialist deity — she is the goddess who contains all feminine powers simultaneously, which is why later traditions found it so easy to identify her with every goddess they encountered.
The Greek historian Plutarch, writing in the 1st century CE, produced the most complete surviving account of the Isis-Osiris myth in his treatise On Isis and Osiris — a text that shaped the Western understanding of Egyptian religion for centuries. By Plutarch's time, the cult of Isis had spread throughout the Roman Empire and Isis had become explicitly identified as the universal goddess — the one divine feminine who appeared under different names in different cultures but was ultimately singular.