EDIT: Many people who are attempting to answer this question are severely misinterpreting the circumstances and setting of the book, so here is some information about Eris that I thought I had included but actually forgot to. This novel is post-apocalyptic. Eris killed her family as a child in an accident, as she possesses the ability to manipulate life force. She blocked out the memory and had convinced herself that she was the last person on Earth until a group of survivors, including Caspian and Marina, arrive and take her in. Eris has literally never known anyone. She cannot remember her family. I am trying to portray her attraction to both boys and girls in the context of her not having any past experience, and only just meeting people that she finds attractive.
My main character, Eris, in my post-apocalyptic novel is queer. Her first love interest, Caspian, is male, but further on in the story I'm going to introduce a secondary love interest, Marina. As far as the reader knows, Eris is straight, because the only person she has expressed romantic interest in is Caspian, a guy. So how can I believably and casually show that Eris swings both ways without the reader being confused by the time she, Marina, and Caspian are in a love triangle?
I want to make clear: this is not sexual. Eris is 16, Caspian is 17, and Marina is either 16 or 17. I will not portray explicit sexual content to show Eris' completely innocent and newly blooming romantic feelings.