So glad you've enjoyed it! The Sixth Sister and that power most definitely play a role in the story, so stay tuned!
Bellona is mentioned as it's the main component in the concoction, the consumption of which grants a person the ability to block and disable magical powers. You might have seen that to some degree in the interactions with Arthur. The flower and the poison will also make frequent appearances in the story later down the line. Another reason is that the legend of the flower's appearance in this world offers some necessary glimpses into the story of the Fourth Sister.