As others said before, the deistic figures come from mythologies of different nations. What is sometimes a deviation of mythological stories are the relations between the figures, at least when we consider Goa'uld and Egyptian mythology.
Most of the facts are "true", for example, Ra really was a rival to Apophis. But wasn't his brother, as said in the series. They were somewhat complementary, representing light and darkness, but it is not said they were brothers. Ra defeated Apophis with great help of Seth. However, because of further events, Seth was really scorned by other gods, as mentioned in series, refer to The Jaffa Joke:
In mythology, Isis and Osiris really formed a couple. This is also true about Ra and Hathor in some versions of the myths. In the series, Ra and Hathor are the parents of Heru-ur (Horus). Most of the time these are Isis and Osiris that are mentioned as his parents, but Ra and Hathor are also mentioned sometimes.
This is in fact hard to make a division between certain deities like Isis and Hathor. Egyptians viewed not only gods, but all existing things as connected, as different formations of one, general substance.
I'm not surprised by any inconsistencies that occur within mythology and the Goa'uld world. If Goa'ulds lied to people about being gods, they also could lie about their relations.
(My main reference at the moment is "Religia starożytnego Egiptu" - "Religion of Ancient Egypt" by Wiesław Bator.)