Why did osiris marry his sister isis?

Answer 1

For Egyptian gods and goddesses, at least Osiris, Isis, Horus, Set, and Nepthys, it is sort of like reincarnation but not. So, originally Osiris, Isis, Horus, Set, and Nepthys were all brother and sister but then later on the story was told differently with Osiris and Set still being brothers and Osiris being married to Isis and Horus being their son while Set and Nepthys were married.

Answer 2

Isis was both Osiris sister and wife. When the story was originally told Osiris and Isis were brother and sister but then later the story was told again and the two were for lack of a better word "reincarnated" being husband and wife.