The nature of the antiferromagnetic order in the heavy fermion metal ${\mathrm{YbRh}}_2{\mathrm{Si}}_2$, its quantum criticality, and superconductivity, which appears at low mK temperatures, remain open questions. We report measurements of the heat capacity over the wide temperature range $180\mu K–80\mathrm{mK}$, using current sensing noise thermometry. In zero magnetic field we observe a remarkably sharp heat capacity anomaly at 1.5 mK, which we identify as an electronuclear transition into a state with spatially modulated electronic magnetic order of maximum amplitude $0.1{\mu }_B$. We also report results of measurements in magnetic fields in the range 0 to 70 mT, applied perpendicular to the $c$ axis, which show eventual suppression of this order. These results demonstrate a coexistence of a large moment antiferromagnet with putative superconductivity.