You don't have enough information yet.
What about the "poly part" is bothering her?
- Is she looking for a traditional mono relationship? (this would be the dealbreaker.)
- Is she afraid to try something new?
- Is she concerned about Poly in general?
- Is she worried about how others might see her?
- Has she met a poly jerk.. and doesn't want anything to do with polys?
And as Ray said... maybe she just needs time to think about it. If it is "just" that she doesn't want to get between you and E, she may need to spend some more time thinking. Then the next time you and E run into her, you can invite her to coffee/casual time together.
Obviously the conversations are delightful/interesting. Let her see your relationship... how it works, how it has worked in the past.