Emergent states in strongly correlated electron systems are typically driven by a close interplay of different kinds of degrees of freedom (DOF), which can be amongst others charge, kinetic energy, spin, or lattice DOF. One interesting example of interrelated, competing order parameters is the interplay of ferromagnetism and charge density wave (CDW) order in SmNiC2, which attracted large attention since Shimomura et al. [1] reported that CDW superstructure modulations are suppressed by the onset of ferromagnetic order. Within the family of RNiC2 compouds, SmNiC2 thus appeared to take an exceptional role. In the present single-crystal study of SmNiC2 grown by the optical floating zone technique, we investigate bulk thermodynamic features of magnetism and CDW phase transitions by means of magnetization, heat capacity and thermal expansion measurements. Else than the earlier suggested Sm2+ to Sm3+ valence cross-over near room temperature, [2] our analysis of the Sm 4f magnetism in terms of crystalline electric field effects is fully consistent with a stable trivalent state of Sm, with its typical features of a finite spin-orbit coupling. Our thermodynamic studies of the first order phase transition, in which CDW order is destroyed by ferromagnetic order, reveal a strong magneto-elastic coupling which is triggered by the competing magnetic and lattice DOF. We discuss our new results obtained on SmNiC2 in the context of general trends of the evolution magnetism and CDW order in the RNiC2 series of magnetic CDW materials with R = Pr - Tm. [1] S. Shimomura et al., Phys. Rev. Lett. 102, 076404 (2009). [2] H. Onodera et al., J. Magn. Magn. Mater. 182, 161 (1998).