  We introduce a first order description of linearized non-minimal ($n=-1$)
supergravity in superspace, using the unconstrained prepotential superfield
instead of the conventionally constrained super one forms. In this description,
after integrating out the connection-like auxiliary superfield of first-order
formalism, the superspace action is expressed in terms of a single superfield
which combines the prepotential and compensator superfields. We use this
description to construct the supersymmetric cubic interaction vertex
$3/2-3/2-1/2$ which describes the electromagnetic interaction between two
non-minimal supergravity multiplets (superspin $\textsf{Y}=3/2$ which contains
a spin 2 and a spin 3/2 particles) and a vector multiplet (superspin
$\textsf{Y}=1/2$ contains a spin 1 and a spin 1/2 particles). Exploring the
trivial symmetries emerging between the two $\textsf{Y}=3/2$ supermultiplets,
we show that this cubic vertex must depend on the vector multiplet superfield
strength. This result generalize previous results for non-supersymmetric
electromagnetic interactions of spin 2 particles. The constructed cubic
interaction generates non-trivial deformations of the gauge transformations.
