We demonstrate that the previously identified L/T transition brown dwarf SDSS J213154.43-011939.3 (SDSS 2131-0119) is a widely separated (82 ''.3, similar to 3830 au) common proper motion companion to the low-mass star NLTT 51469, which we reveal to be a close binary itself, separated by 0 ''.64 +/- 0 ''.01 (similar to 30 au). We find the proper motion of SDSS 2131-0119 of mu(alpha) cos delta = -100 +/- 20 mas yr(-1) and mu(delta) = -230 +/- 20 mas yr(-1) consistent with the proper motion of the primary provided by Gaia DR2: mu(alpha) cos delta = -95.49 +/- 0.96 mas yr(-1) and mu(delta) = -239.38 +/- 0.96 mas yr(-1). Based on optical and near-infrared spectroscopy, we classify the primary NLTT 51469A as an M3 +/- 1 dwarf, estimate photometrically the spectral type of its close companion NLTT 51469B at similar to M6, and confirm the spectral type of the brown dwarf to be L9 +/- 1. Using radial velocity, proper motion, and parallax, we derived the UVW Galactic space velocities of NLTT 51469A, showing that the system does not belong to any known young stellar moving group. The high V, W velocities, lack of a 670.8 nm Li I absorption line, and absence of H alpha emission, detected X-rays, or UV excess, indicate that the system is likely a member of the thin disc population and is older than 1 Gyr. For the parallactic distance of 46.6 +/- 1.6 pc from Gaia DR2, we determined luminosities of -1.50(-0.04)(+0.02) and -4.4 +/- 0.1 dex of the M3 and L9, respectively. Considering the spectrophotometric estimation, which yields a slightly lower distance of 34(-13)(+10) pc, the obtained luminosities are -1.78(-0.04)(+0.02) and -4.7(-0.5)(+0.3) dex. We also estimated their effective temperatures and masses, and obtained 3410(-210)(+140) K and 0.42 +/- 0.02 M-circle dot for the primary, and 1400-1650K and 0.05-0.07 M-circle dot for the wide companion. For the similar to M6 component, we estimated T-eff = 2850 +/- 200 K and m = 0.10(-0.01)(+0.06) M-circle dot.