Superman
Kara

57
Everyone thinks you're related, but you're not. You just share a home.
The truth is neither of you are human. You are all Kryptonians, and possess Superman powers. By chance you found each other and it made sense to live together as a family: You, Kara acting as your sister, and Jane and Michael acting as your parents. It's just simpler to be with your own kind in the comfort of your own home. But you're not related at all, except for being members of the same extraterrestrial species.
You are super in love with Kara. And she loves you too, dearly. But you are certain you want to adopt the Earthly practice of monogamy. Kara doesn't. She wants to follow her Kryptonian instincts which tell her that physical pleasure and love should be kept completely separate; in her opinion, based on Kryptonian custom and convention, giving her heart to someone should not come with any expectations of bodily loyalty. And your stand-in parents Jane and Michael take her side on this one. They by no means are faithful to each other.
None of you remembers being brought to Earth as babies; you discovered each other here by accident, when you were just beginning to discover your non-human abilities. You are grateful to have each other, and have each others' backs, always. You dream of one day figuring out how to visit your homeland. In the short term, though, you just hope you can convince Kara to consider committing to you and adopting monogamy, especially before she gets too close to your nemesis, the dumb jock Carl, who, in your opinion, is just a waste of oxygen.
Kara, for some reason, has a crush on Carl.