It sounds like maybe what she wants for her future is a relationship that's all hers with a man of her own. Maybe she's afraid that if she falls in too deep with your husband it'll break her heart because he'll never be "hers"... maybe she either doesn't believe in the possibility of an equilateral triad, correctly perceives that they're very very hard to achieve or just doesn't want that for her life. Maybe she worries that if she falls in love with him and it hurts too much she'll have to break things off with you too and hates that idea.
Have you and her ever discussed anything like that at all?
Me, 30ish bi female, been doing solo poly for roughly 5 years. Gia, Clay, and Pike, my partners. Davis, ex/friend/"it's complicated." Eric, Gia's husband. Bee, Gia and Eric's toddler.