Ah interesting.
A Story young man who summoned one to be his Big Titty Slutty Goth Girlfriend and brought it home where during the night it looked around this sexually charged house and thought why not, no influencing the Father and the Girls in the house who no have only the father figure as the only adult in their lives after the mother of the group leaves for a period of time. AND at the time who happens to be a figure of sexuality for them as well considering that we haven't been shown that these girls have any other male intrests or bi/les intrests, though with your choices it looks like you might be able to.
And instead of being a case of one trying to get rid of the other, the sucubus instead in working on both males to get the most out of the two of them. This will lead to a Father Son teamwork on corrupting the women in their lives and feeding the sucubus for a long time.
Note, This is what I took away from the story, I am not in any know to know if its fact, but this is what it is to me.