I might be overstating it some with slapping a "toxic" label on it; "doomed" might be a more apt description.
Foremost, Seraphine (the astropath) places Lisbeth (the Sororitas) up on a pedestal. All of her own worth and self-image is stocked into how Lisbeth receives her. She feels as if she does not exist unless she is acknowledged beneath her gaze, for all of her heart belongs to Lisbeth. However, Lisbeth feels somewhat responsible for her as a savior, but she is off-put by her mutation (psyker-ism) and frequently discounts her station; viewing her more civilian despite her role as Choir Mistress aboard a RG vessel. For all the barbs she carries against her, Lisbeth discretely seeks her affection and has no qualm in purloining it.
The central wedge between them is that Lisbeth's heart totally belongs to serving the God-Emperor. He is quite literally her reason of being and she never falters from steeling her heart to Him. Seraphine seeks purpose, ardently, and sequesters all of it in Lisbeth — she is Seraphine's idol, momentum, raison d'etre, etc. She does everything in her power to be as pious as her, as god-fearing as her.
Lisbeth is in no way deserving of it. She and Seraphine met when they were young (teenagers) and spent a season in tutelage together after acolyte Lisbeth pulled Seraphine from the rubble of her first station. Seraphine learning of Lisbeth's pure, giving, and dutiful heart, began to immensely admire her strength and tenacity. It eventually blossoms into a form of obsession, removing the human component of Lisbeth to instate her image to that of a Saint in her mind. Lisbeth is no better, minimizing Seraphine to the young delicate girl she entreated with lessons in hymnal, prayer, and tutored through the halls of their Cathedral.
TL;DR : Lisbeth's heart belongs to the God-Emperor and battle, yet she indulges in Seraphine against her better interests. Seraphine erases herself of personhood to be appealing only for Lisbeth; to feel wanted, cherished outside of her duties.
