I feel like we want to be using as simple concepts and language as possible. We really don’t want to be adding anything more for the audience to be thinking about than the bare minimum to make this work.

Parents joined the cult? Then let’s just say that directly in simplest terms. We don’t need to delve deep into her parents’ motivations beyond carrying this bit of plot.

In simplest terms, to carry us from “nobody parents” through “Rey Palpatine” to the events depicted in TROS:

Rey is one of many experiments
Her nobody ‘parents’ joined the Sith cult
They became her parents more truly (implicit adoption through love-inspired kidnapping)
They left her on Jakku (+link to Luke via implicitly Lor San Tekka)
Palpatine wanted to kill her as a powerful potential rival
One of Palpatine’s assassins (with implicit connections to Exegol) failed to kill her, but did kill her parents
Her parents died as nobodies once more
Palpatine still wants her dead until realising just how powerful she is- perhaps a better vessel than his current one
Ultimately, it’s the power of the Dyad which reveals itself as an unexpected way to heal his current body, rendering her killable once more