The Dragon Queen by Michelle Madow

The final battle is here.

Prepare yourself for a mind-bending adventure full of magic, romance, and twists you’ll never see coming.
Gemma and her soulmate Ethan have remembered the true past they shared together. So everything in her life should be perfect. Right?
Wrong. Gemma’s worst nightmare has come true. Her twin sister Mira has gone dark and joined forces with Lilith and the demons.
Gemma has already learned the hard way that she can’t use her time travel magic to save her sister. But she can use it to help defeat the demons.
The best way to do that? Find the only Dark Weapon the demons don’t have yet—the Dark Sword—and use it against them.
But the Dark Sword is lost in time—over a century in time, to be exact. Now, it’s up to Gemma to find it and bring it to the present before Mira can get it first. And if Mira does find it first, Gemma will have to do everything she can to steal it from her, even if it means committing the ultimate act of betrayal: turning on her twin once and for all.