Cashew, mango, pistachio, and poison ivy are all part of the evergreen tree family. Mango also contains a fruit protein which is also contained in apple, pear, and peach. Due to these cross-reactive proteins there is a possibility of these foods having a family effect.
Patients who are cashew allergic typically will be clinically allergic to pistachio and less likely allergic to mango. While allergy testing can give some predictive value the only way to truly prove allergy is oral challenge. Obviously in most cases this would be needed to be done under close supervision.