The answer to this question depends on which source material you're referencing.
In Egyptian mythology, Nephthys and Seth are siblings who were married. However, the story of their relationship is often told in different ways, with varying degrees of detail and complexity. Some versions of the myth suggest that Nephthys had a child with Osiris, Seth's brother, who was also married to Isis.
This could be interpreted as Nephthys cheating on Seth, but it's important to remember that the ancient Egyptians had a complex understanding of marriage and family, and their mythology often reflected this. It's possible that the story of Nephthys and Osiris is meant to be symbolic rather than literal, representing the struggle between order and chaos, or the tension between different deities.
Ultimately, whether or not Nephthys cheated on Seth is open to interpretation. It's up to each individual to decide how they understand the story and its implications.
