Writing UI tests manually requires significant effort. Several approaches have tried to address this problem in mobile apps: by exploiting the similarities of different apps within the same domain on a single platform, they have shown that it is possible to transfer tests that exercise similar functionality between the apps. A related recent technique enables transfer of UI tests uni-directionally, from an open-source iOS app to the same app implemented for Android. This paper presents MAPIT, a technique that expands existing work in three important ways: (1) it enables bi-directional UI test transfer between pairs of "sibling" Android and iOS apps; (2) it does not assume that the apps’ source code is available; (3) it is capable of transferring tests containing oracles in addition to UI events. MAPIT runs existing tests on a "source" app and builds a partial model of the app corresponding to each test. The model comprises the app’s screenshots, obtainable properties of each screenshot’s constituent elements, and labeled transitions between the screenshots. MAPIT uses this model to determine the corresponding information on the "target" app and generates an equivalent test, via a novel approach that leverages computer vision and NLP. Our evaluation on a diverse set of widely used, closed-source sibling Android and iOS apps shows that MAPIT is feasible, accurate, and useful in transferring UI tests across platforms.