A well-defined nodule is generally considered a feature more indicative of a benign lesion. This could very well be a cyst or a Fibroadenoma (Both benign). However the Radiologist also examines other features, such as vascularity, shadowing etc., which need to be evaluated to form a complete assessment of the lump. If you and your wife are overly concerned, because of her family history of breast cancer, you could discuss with her Doctor if a fine needle aspiration could be needed for pathology study.
If a biopsy confirms that it's indeed a simple cyst or Fibroadenoma, then you and your wife have nothing to worry about.