I killed Harkron and I just remembered that I must free her mom after that. I went to the soul cairn but she's nowhere to be found. Help.

Go back to where you first found her

From the entrance to the Soul Cairn, just head straight back in as direct of a line as you possibly can. She should be waiting for you either on her doorstep, or in the Arena-like enclosure where you fought Durnhaviir.

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I did. She wasn't there

Reload from your save before entering the Soul Cairn to go back and get her. I think it's a glitch, same type as why people aren't where they're supposed to be in vanilla Skyrim.

She should be there on a reload. If not, check around the entrance just inside the Soul Cairn. Alternatively, check to make sure she isn't inside the courtyard of Castle Volikhar or in her Alchemy Lab.

No I already met her. She said she could never go backto tamriel with Harkron alive.I killed him re entered and can't find her.

Then she may be wandering the Castle already...

,......oh........well....is this confirrmed

Not confirmed, no... maybe you should go look? lol