I haven't read the WN or LN yet, but here is my guess relating to the plot of the story.
The Goddess(?) people worship is actually herself a twin, but her other twin was more powerful and capable than her, which caused her to be extremely jealous of her twin. So, when there was a war, the Goddess taking the chance, either killed/imprisoned her twin, and spread the fake information.
It is also possible, that there isn't a Goddess, but instead it was a normal person who had a twin, and the said twin was more powerful and capable than the other (the rest would be same as the scenario above).