We study close-to-equilibrium properties of a one-dimensional symmetric inclusion process (SIP) with finite size by coupling it to two particle reservoirs at the two boundaries with slightly different chemical potentials. The boundaries introduce irreversibility and induce a weak particle current in the system. We calculate the McLennan ensemble for the SIP, which corresponds to the entropy production, and the first-order nonequilibrium correction for the stationary state. We find that the first-order correction is a product measure and is exactly consistent with the local equilibrium measure corresponding to the steady-state density profile in the finite-size SIP, without the need to be in the thermodynamic limit. This provides a novel example for microscopic extensions of the McLennan formula and the interpretation of first-order nonequilibrium correction as entropy production.