While Domain-Specific Modeling Languages (DSMLs) are increasingly used in industry, both their definition and tooling (e.g., checkers, document or code generators, model transformations) still require significant development efforts that must be balanced with their limited number of users (by definition). Unfortunately, the current Model-Driven Engineering (MDE) technologies strongly rely on the conformance relation that bind a model to the unique DSML used to create it. Consequently, while most of the tools would be reusable for a family of close DSMLs, in practice it is not possible. In this paper, we propose to abstract the overly restrictive conformance relation with a typing relation allowing to manipulate a model through different DSMLs and uncover the need for model-oriented type systems. We introduce K3SLE a new modeling framework built on top of the Eclipse Modeling Framework (EMF) that leverages family polymorphism to support the typing relation. Based on the typing relation, it natively provides model polymorphism, language inheritance and DSML evolution and interoperability. We demonstrate its use on representative use cases.