This is how families work. No matter how much the person hurts you, you can’t just cut them off completely. And I am saying this from experience. My maternal uncle is an alcoholic, who verbally abuses his sisters, violent toward his parents and blames them all for everything wrong in his life, but his family still cares for him, they always go running to help him when he is in trouble.
Her character really isn’t stupid at all. She understands that Alicia will never change but her trauma on watching all her siblings die has obviously had a tremendous impact on her life to the point she believes she shouldn’t be able to accept Vincent’s love. She regrets being one of the only children out of all her siblings to live, so of course she’s going to do whatever it takes to keep her last sister alive whether she’s evil or not.

Is she insane? She still cares about that evil sister? She's crazy