I have 2 tables exposed through a Feature Service tha has a M-N relationship. Assuming that I have a Graphic object from each table, is it possible to create a relationship between the two?
Your tables can be represented as FeatureLayer, where Graphic.Attributes represent the row in the table. This feature service for example are related by an attribute "sf_311_serviceoid" (i.e. "2547813"): http://sampleserver3.arcgisonline.com/ArcGIS/rest/services/SanFrancisco/311Incidents/FeatureServer. To relate graphics then, you can update their attribute value. But you need to know which attribute connects the two tables.