They were relatively close and worked together to make a possessed skaab, a body coppled together from corpses with a human soul to guide it. They succeeded, but their creation went berserk and killed Elaine. Ever since, Marienne’s been obsessed with finishing their work to bring her sister. She’s been studying her sisters notes on geist-magic, and she thinks she can finally do it.

I think she's also a bit of a geistmage, and I think her true goal is to bring someone back.