that's her mother and father, no remarriage in this manga.
from the scenes we've seen, the father made a 'move' on the younger daughter, but that's from the point of view of the heroine
the father never made a move on the older sister but she thinks he is ignoring her and paying too much attention to her younger sister, thus she does not belong home
probably I'd say she is an attention and thrill seeker, she wants to be noticed, appreciated and that turns her on, nothing unnatural for any teenager...