As Robert Scheitlin, GISP mentioned, currently only relationshipclasses for the first level are support in WAB. You can access them through REST, but the widgets in WAB wont show them.
We had a simular case and had to reduce the relationshipclasses to a single level (some attributes were added to the featureclass and the rest in the 1:M related table).
There are some other restrictions when consulting related features. From a selection of the table you wont be able to show the related features or filter features based on a selection from the related table. To mimic this functionality we tested converting the table to featureclass (using the label points of the related polygons). Far from optimal, but could be an option in some cases.