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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?

So how can I believably and casually show that Eris swings both ways ...

Casually! Have a conversation between Eris and Caspian, perhaps in bed, talking about sex.

An Example:

Caspian asked, "When was your first time? Like, full on?"

"With a girl, I guess full on means oral, so I was fifteen, I had this thing with a friend at school. Then she got religion and dumped me. Not with a guy until I was seventeen. I mean, I was willing, but no good candidates. And you?"

"When I was eighteen, with Elly. You know her?"

"Know her?" Eris laughed. "I would totally fuck that girl. Was she any good?"

"Yeah, I mean, we didn't know anything, but yeah. Very ... energetic."

Eris saw him grinning, and laughed again.

The conversation is casual pillow talk, Caspian is not surprised Eris is bisexual, Eris doesn't hesitate to reveal it.

I don't think you need to show anything special at all.

Lots of people have multiple love interests (or hookups) over the course of a novel. In some novels, it's the premise.

If a character's first relationship in the novel was to a tall blond German runner, you wouldn't think your readers would be confused when the next relationship is with a short bald Nigerian physics professor.

Let your reader be confused. Most readers will figure it out pretty quickly. The few that don't, well, they're the readers that wouldn't really get it after you explained it either.

You can add subtle hints in the way Eris interacts with the other women in the story. It is not entirely unusual for women in our current culture to compliment each other's looks. You can add a very slight tint of sexuality to act as a hint.

Something like:

That shirt really makes your boobs look good.

(I'm a straight man, so I may not have the best voice for a bisexual girl and it's a post-appocolyptic world so they probably aren't talking about fashion, but you get the idea)