System families are an idea of software reuse in a specific problem domain. Existing methods have little requirements engineering support for system family development. This short paper proposes a requirements metamodel for system family development. Traceability throughout model elements is a necessary precondition for preserving the consistency of the complete family model during development and is a main issue in this paper as well as for software development in general. Family development based on the metamodel guarantees traceability by the inclusion of all development artifacts in a single and consistent model. Keywords System families, traceability, software reuse