There's only one story that springs to mind, but it is from the Greek occupation period: Plutarch writes that Nephthys, sister of Isis, was married to Seth, but since this deity is associated with infertility, this marriage produced no children.
According to some versions of the Osiris myth, Nephthys intoxicated Osiris, her sister's husband, to conceive Anubis. In other versions, Anubis is the illegitimate son of Ra and Nephthys.
Plutarch also writes that Isis adopted her sister's illegitimate son:
For when Isis found out that Osiris loved her sister and had relations
with her in mistaking her sister for herself, and when she saw a proof
of it in the form of a garland of clover that he had left to Nephthys
- she was looking for a baby, because Nephthys abandoned it at once after it had been born for fear of Seth; and when Isis found the baby
helped by the dogs which with great difficulties lead her there, she
raised him and he became her guard and ally by the name of Anubis.
However, since all these stories date from the Greek occupation, they are possibly "contaminations" from the Greek pantheon, as in earlier periods, infidelity was very much a taboo in Ancient Egypt, although, as in most cultures, men were given much more lenience than women in this matter.
