Well, first Isis and Osiris were brother and sister then when the story was told again it was told as if the two were in another life being husband and wife. So, I guess that the reason they got married was because the person who told this version of the story probably thought that either Isis or Osiris had contacted them and told them this because the person who did say this was either a priest or a priestess.
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Her Husband was her brother Osiris. In those Ancient times brother and sisters could of gotten married to each other.
The lover of Isis was Osiris.
Horus was the son of Osiris and Isis. But in some other stories these three are depicted as brothers and sister.
Osiris
osiris,isis seth and nepthys