The present work demonstrates the importance of multiple structures and torsional anharmonicity in determining the thermodynamic properties of branched open chain silicon hydride clusters (SinHm and SinHm-), which are important constituents in nanodusty silane plasmas. We include all categories of such single-bonded species for n = 5 and 6, namely silyl radicals, silyl anions, silylenes and silylene anions, and silanes. We calculated the statistical mechanical partition functions by employing the multi-structural (MS) method for torsional anharmonicity in combination with density functional theory. The partition functions were then used to estimate thermodynamic quantities, in particular Gibbs free energy, enthalpy, entropy, and heat capacity. The calculations included contributions from all conformational structures of all possible isomers of each species and were performed in three ways, namely by using the multi-structural quasiharmonic approximation (MS-QH), the multi-structural method with uncoupled torsional potential anharmonicity (MS-T(U)), the and multi-structural method with coupled torsional potential anharmonicity (MS-T(C)). Our results indicate that the entropic effects on the thermochemistry are large and are primarily dominated by multi-structural effects, although torsional potential anharmonicities are not negligible. We find that the multiple-structure effect, which is always greater than unity, is not only very large (as large as a factor of 592) but also very isomer dependent, so that free energy differences between isomers can be greatly affected. The torsional potential effect is relatively smaller on average but certainly not negligible; it varies from a factor of 0.2 to 1.4.