Thanks for the clarifications. I agree with Annabel that it sounds like you're doing a good job at discussing everything between you and your husband and approaching the growing relationship well. I suggest that some of these topics be broached with your husband's new partner as well. For example, discussing what to do if she wants to move in with you in future because she's currently living with her parents may be a moot point if that's not something she's actually interested in.
As far as her finding it strange that you're accepting of the relationship your husband has with her, it's ok to give her time to get used to that idea. If she's coming from a background with no poly experience she will need time to adjust her thoughts. Perhaps sharing some books or websites with her would help. My husband shared 'Opening Up' with his new partner recently and she really enjoyed it and said it helped her understand the new kind of relationship we were sharing with her.
Good luck to you and yours