The three pentaquark states, $P_{c}(4312)$, $P_{c}(4440)$, and $P_{c}(4457)$, discovered by the LHCb Collaboration in 2019, can be nicely arranged into a multiplet of $bar{D}^{(ast)}Sigma_{c}^{(ast)}$ of seven molecules dictated by heavy quark spin symmetry. In this work we employ the effective Lagrangian approach to investigate the two decay modes of $P_{c}(4457)$, $P_{c}(4457) to P_{c}(4312) pi$ and $P_{c}(4457) to P_{c}(4312) gamma$, via the triangle mechanism, assuming that $P_{c}(4457)$ and $P_{c}(4312)$ are $bar{D}^{ast}Sigma_{c}$ and $bar{D}Sigma_{c}$ bound states but the spin of $P_{c}(4457)$ can be either 1/2 or 3/2. Our results show that the spin of $P_{c}(4457)$ can not be discriminated through these two decay modes. The decay widths of $P_{c}(4457) to P_{c}(4312) pi$ and $P_{c}(4457) to P_{c}(4312) gamma$ are estimated to be of order of 100 keV and 1 keV, respectively. The ratio of the partial decay widths of $P_{c}(4457) to P_{c}(4312) pi$ to $P_{c}(4457) to P_{c}(4312) gamma$ is similar to the ratio of $D^{ast}to Dpi$ to $D^{ast}to Dgamma$, which could be used to check the molecular nature of $P_{c}(4457)$ and $P_{c}(4312)$ if they can be observed in the future.
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