Somebody remind me, wasn't Lucoa's entire Huge Embarrassing Secret™ that she got white-girl-wasted and slept with her sister?
What exactly was the embarassing part? From the bit we've seen of Petla, it doesn't feel like she'd really mind, you know?
If I remember correctly there was some sort of cursed sake involved.
The question now is: Is Petla this way as an effect of said cursed sake or has she always been like that? And if it's the latter, was Lucoa kicked from her Goddess position because it was public and not because it happened?
Based on the early chapters Lucoa always thought that Petla hated Lucoa for "raping" her, but latest chapters revealed that Petla was into the whole thing and it turned her into an Incesiscon.
Lucoa then used the excuse of being Shouta's familiar to turn down Petla's attempts to get her Yuri-Pregnant, so Petla decides to become the second familiar to get an in through a future threesome.
In the original myth, or at least the one the author is pulling from, Quetazalcoatl was given pulque (traditional alcoholic drink) by his brother Tezcatlipoca, in some versions he told him it was medicine as Quetzalcoatl was a bit sick at that time, some versions said it was 2 other gods who wanted Quetzalcoatl to forget his troubles. The reason they gave him the pulque to begin with varies between interpretations, some say Tezcatlipoca was jealous of Quetzalcoatl's power and divinity as he was akin to a high priest doing penitence and following vows, so Tezcatlipoca wanted to corrupt him. Once Quetzalcoatl started drinking he called for his sister, Quetzalpetlatl (who was also akin to a high priestess), to join him, both of then got seriously drunk and lost their inhibitions resulting in them having sex. Ashamed for what had happened, Quetzalcoatl exiled himself to reflect on his behavior.
In no version I could find was it implied that it was rape, more so that alcohol made both of them have sex, it's more so a tale about alcoholism.
This is kind of an obscure myth, as Quetzalcoatl is commonly depicted with having only brothers. Quetzalpetlatl is rarely mentioned or doesnt even exist. The myth in question came from the Codex of Chimalpopoca which got lost back in the 1940s, there's some surviving copies but it's not really certain if they are the same as the original one, some of them are even considered fake.
In Kobayashi it's a little less clear what really happened, the beats are similar however, Lucoa got drunk (with pulque more likely than sake) and had sex with Petla. In ch 137 Lucoa said Petla was really mad immediately after they had sex. But enough time passed that Petla forgave her and missed her, also she got jealous of Shouta hence why she is now his familiar too, just to be with Lucoa and to get back at Shouta for being with her sister.
Hope that cleared things up.