Bi-curious would be the right word.
I’d say both bisexual and bicurious fit Holly. She’s clearly bisexual in her feelings for Lena on that route, while her unrequited feelings for Ian are still there after what happened between them early in the game. And of course, the three-way relationship with Ian, Holly and Lena also fits within that context.
On the Ivy and Lena route, she’s more in a phase of exploring her sexuality, so she’s bicurious at that point. During that time, she’s open to being with girls but still spends time with guys who seem to remain her main focus, at least that’s how I see it.
To me, the term bicurious implies more of a slight attraction or sexual openness toward people of the same sex, while bisexuality involves both sexual and romantic attraction to both sexes without distinction.