Theia is skeptical of some aspects of the death eaters, not fully indoctrinated, despite her fiance being one, so her sister being more of a blind follower sounds good! It would make her more critical of the safety of being a follower of Voldemort as she would stand to lose two of her favourite people.
Haha, I'll bet their mother will move right on to wanting her to get betrothed so she can do the pureblood marriage and wedding right with one child before Theia can corrupt her.