Young brown dwarfs serve as analogues of giant planets and provide benchmarks for atmospheric and formation models. JWST has enabled access to near-infrared spectra of brown dwarfs with unprecedented sensitivity. We aim to constrain their chemical compositions, temperature structures, isotopic ratios, and disc emission. We perform retrievals and disc modelling on JWST/NIRSpec medium-resolution (R 2700) spectra spanning 0. 97--5. 27 μm, combining radiative transfer, line-by-line opacities, parameterised temperature profiles, and flexible equilibrium chemistry. We include a disc ring with blackbody continuum and optically thin CO emission. We detect over twenty species, including ^12CO, H₂O, CO₂, SiO, and hydrides. The CO band at 4. 6 μm reveals ^13CO and C^18O. Carbon isotope ratios are ^12C/^13C = 79^+14-₁₁ (TWA 27A) and 75^+2-₂ (TWA 28) ; oxygen ratios are ^16O/^18O = 645^+80-₇₀ and 681^+53-₅₀. Both objects show excess infrared emission, consistent with warm (650 K) blackbody rings, and optically thin CO from hot gas (1600 K) needed to match the red spectra. The atmospheric C/O ratios are 0. 54 0. 02 (TWA 27A) and 0. 59 0. 02 (TWA 28), consistent with solar values. We characterise the atmospheres and discs of two young brown dwarfs through joint constraints on temperature, composition, isotopes, and discs, demonstrating JWST/NIRSpec's ability to probe young objects and circumplanetary discs.
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