My theory into what happened to Celine.

If there is one I wasn't a big fan of in the movie, despite how great the movie was, was how after Rumi finally comes to terms with her demonic heritage, despite the pleas of Celine, we don't see the latter again and I'm hoping the sequel will address it but here is my theory into what happened.

After no longer feeling afraid of who truly is and with the love and support of Mira and Zoey, Rumi realizes the harm Celine was doing regarding how she was hiding her markings and therefore, her identity and despite how Celine raised her like a daughter, she simply did what anyone would do with any other toxic family member, she essentially kicked Celine out of her life and ensured to never come close to her and the girls ever again.

As for the aftermath? I imagine Celine would be saddened and betrayed by a girl she raised as her own and given her hope is to eradicate demons onto earth, she now sees Rumi embracing the demonic side of her as a danger and she now wants to stop her, no matter what.

As I said, the sequel may present something very different but that's what I believe happened with Celine.