Excitation functions and angular distributions of the reactions $^{28,30}\mathrm{Si}$($\ensuremath{\alpha}$,${\ensuremath{\gamma}}_{0}$) were used to study the distribution of $E2$ strength in the energy regions 11-21 MeV of $^{32,34}\mathrm{Si}$, as well as to probe the importance of isospin in the decay of the $E1$ giant resonance in these nuclei. It was found that the $E2$ strength in the ($\ensuremath{\gamma}$, ${\ensuremath{\alpha}}_{0}$) channel is widely distributed and accounts for about 12% of the energy weighted isoscalar sum rule in each nucleus. Together with the $E2$ strength observed in lower resonances, about 45% of the sum rule is accounted for in $^{32}\mathrm{S}$ and 34% in $^{34}\mathrm{S}$, where the measurements on the lower resonances are incomplete. The spreading of the $E2$ strength can be attributed to the mixing of $n\mathrm{p}\ensuremath{-}n\mathrm{h}$ configurations into the basic 1p-1h excitations of the $E2$ resonance, and the large $E2$ strength is attributed to the presence of a direct or semidirect component in the ($\ensuremath{\gamma}$,${\ensuremath{\alpha}}_{0}$) process. A comparison of the $E1$ strengths in the ($\ensuremath{\gamma}$,${\ensuremath{\alpha}}_{0}$) channel of the giant dipole resonances in $^{32,34}\mathrm{S}$ indicates that isospin conservation is important in these reactions. The relative weakness of the $E1$ ($\ensuremath{\gamma}$,${\ensuremath{\alpha}}_{0}$) strength in $^{34}\mathrm{S}$ compared to the $E2$ strength is attributed to the relative purity of the 1p-1h character of the $E1$ resonance as compared to that of the $E2$ resonance.NUCLEAR REACTIONS $^{28}\mathrm{Si}$($\ensuremath{\alpha}$,${\ensuremath{\gamma}}_{0}$), $E=5\ensuremath{-}16$ MeV; $^{30}\mathrm{Si}$($\ensuremath{\alpha}$,${\ensuremath{\gamma}}_{0}$), $E=4\ensuremath{-}15$ MeV; measured $\ensuremath{\sigma}(E,{E}_{\ensuremath{\gamma}},{\ensuremath{\bigominus}}_{\ensuremath{\gamma}})$. $^{32,34}\mathrm{S}$ deduced $E1$, $E2$ strengths. Enriched targets.