You should think of some specific things to ask her to do (e.g. let you make his thermoses) that would ease your concerns. Ask her directly (in person would probably be best). Remember, communication works best when it doesn't contain implied accusations (or "tough talk" like "I don't like your attitude"). Instead, stick to describing your feelings as needed (while owning those feelings and not blaming anyone else for them), asking for specific things that would help you, and bracing yourself to "be the better person" in case she doesn't handle it well. As the saying goes, it's more important to be kind than to be right.
You may need to approach Hubby too, and tell him that you need him to understand that you do have worries about being replaced. See if there are any specific things he can do to ease your concerns, and ask him directly. Sometimes the other person's answer is "No," but then you try to negotiate a compromise.
Love means never having to say, "Put down that meat cleaver!"