She’s void imbued red dragon (she call Alex sister). Her troops using twilight abilities, but they look slightly different. As Vexion herself.
Looks like it’s somewhat same as twilight dragon, but made differently. Not through third party like experiments by Sintharia, but directly fueled by void powers from old gods.
Yven, iirc, is convinced she’s the echo of Ysera from the aftermath of the Emerald Nightmare raid, corrupted by the void flower we left behind the first time.
She’s not really hiding, if she’s calling Alex “sister”. So far as we know, she only had the one.
Though, my hope is that it’s some thing the Reds do, like Night Elves, where they call each other brother and sister regardless of biological relation.
Also i wonder, since we didn’t kill her now - what will she be? A Raid boss or just will be killed in other quests? Model does look unique enough to be a boss.
Corrupted Dream of Ysera might be willing to boast of being a Dream Construct created by Ysera in her own image while taking on her own name to signify her freedom from the real Ysera.
If the notion that Dream of Ysera is not the real Ysera’s ghost surprises you remember the Statue of Alexstrasza who flew Krasus and Malfurion back to the camp to aid the Wild Gods against the Legion in the War of the Ancients Trilogy.
It’s probably a family quirk to for Alexstrasza and Ysera to create copies of themselves that see each other the same way the real ones do. Alexstrasza even admits that her statue would treat Korialstrasz the same way she would I.E. as her favored Consort.
I would take this being true over some explanation of how the void stole her soul from elune and corrupted it. She was ready the easiest to corrupt dragon it seemed.
Alexstraza does not acknowledge Vexion as her sister though, or as any relation to her.
Vexion acknowledges Sinestra as her mother, so she isn’t really related to Alexstraza and is just using the term ‘sister’ to try and assert herself as the equal of the queen of the red dragon flight.