She's now arachnophobic and hates the smell of incense, which reminds her of her mother.

As an initiation rite, Nyx was forced by her mother to kill her father as a sacrifice to Lolth, but she refused: that lead to their father being killed by their mother, and Nyxs act of cowardice caused her to be banished from her clan with Nox.

They didn't grow up together by their mother's orders, but they sneaked out to see each other once every week (during one sneak out they crossed paths with the Selunites procession).

On a more specific note (abt Nyx):
Their mother was a cleric of Lolth, and Nyx was raised to be one too (which might explain why Nyx became a cleric and Nox didn't), while Nox was looked down by everyone.

But then the orphanage gets attacked by sharites and burns to the ground, leaving them with no home nor family: Nyx is now in search of vengeance, while Nox is looking after her sister while trying to understand the upper world.

Being kicked out of their clan, they seek help to a selunite orphanage, which raises them as devote followers of Selune.
While Nyx gets divine powers from her devotion, Nox doesn’t, leading to him focusing on music instead.