In Chapter 13 in the Special Events section, the book says Patrina's ultimate desire is to become Strahd's betrothed, with a secondary desire to avenge her brother's injury at the hands of Rahadin. If/when the party and Kasimir finish the Amber Temple and then go back to the Castle's catacombs to raise Patrina... she will discover that there are currently at minimum four other vampire spawn who are Strahd's brides (and husband)-- Ludmilla Vilisevic, Anastrasya Karelova, Volenta Popofsky and Escher-- in addition to a young girl Gertruda who is also a potential mate inside the castle. There is also a crypt with another old wife of Strahd's, Sasha Ivliskova.
So her very first thing she'd want to do would be to eliminate EVERYONE who is currently mated to Strahd, because only then could she become his one true consort. So she could easily work with the party to accomplish their quests so long as they help her kill and destroy all of Strahd's other consorts and Rahadin as well. That includes killing the spawn, and staking their bodies to destroy them fully. If Ireena Kolyana is still around, then she too will be a target of Patrina (as Strahd has eyes for her), and now the party has to decide if they help Patrina kill Ireena too.
Basically, you can set it up where Patrina is realized by the party as an extremely strong and competent wizard, who has quite possibly more power than any one of them do. So they'll want to help her out to keep her on their side and help them accomplish their goals... but soon enough discover the cost of NOT helping her accomplish her own. It becomes a Catch-22... do they help her destroy all these consorts and suitors of Strahd, even if some of them are not evil or worthy of the violent death she would bring upon them? Or do they try and stop her madness, and thereby potentially make enemies of Kasamir and his Vistani?
If this particular plot of Patrina slaughtering all potential suitors drives forward... you can start introducing more and more women and men throughout Barovia, Vallaki, and Kresk that Patrina comes to think are would-be consorts and goes on a blood rampage to destroy all of them. The Bride in Kresk's abbey... Milivoj in St. Andral's Church... Ezmerelda d'Avenir... and if you want to push things even further Patrina comes to believe even the baby Yolanda at the Wizard of Wines is going to grow up to be one. Anyone and everyone could become a Strahd suitor in her eyes at some point in the future, and thus see needs to eliminate them all. The question just becomes... how far along does the party help her on her quest before they realize what she (and they) have now become?
