I think the series is utter drek and I was ready to slit my wrists after sitting through the first movie with my daughter.
To speak to your question, though, sort of: I wonder why either one of them would want her? She's one of the most shallow characters ever created! Why would a 100+ year old man have any interest in such a horridly immature girl?
Of course, he's a right creepy old bastard, but I'm not going to pursue that here.
When speaking of various forms of non-monogamy...it ain't poly if you're just fucking around.
While polyamory, open relationships, and swinging are all distinctly different approaches to non-monogamy, they are not mutually exlusive. Folks can, and some do, engage in more than one of them at a time--and it's all good.