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The jet breaks in the afterglow lightcurves of short gamma-ray bursts(SGRBs), rarely detected so far, are crucial for estimating the half-openingangles of the ejecta ($\theta_{\rm j}$) and hence the neutron star merger rate.In this work we report the detection of jet decline behaviors in GRB 150424Aand GRB 160821B and find $\theta_{\rm j}\sim 0.1$ rad. Together with fiveevents reported before 2015 and other three "identified" recently (GRB 050709,GRB 060614 and GRB 140903A), we have a sample consisting of nine SGRBs and onelong-short GRB with reasonably estimated $\theta_{\rm j}$. In particular, three{\it Swift} bursts in the sample have redshifts $z\leq 0.2$, with which weestimate the local neutron star merger rate density {to be $\sim1109^{+1432}_{-657}~{\rm Gpc^{-3}~yr^{-1}}$ or $162^{+140}_{-83} {\rmGpc^{-3}yr^{-1}}$ if the narrowly-beamed GRB 061201 is excluded}. Inspired bythe typical $\theta_{\rm j}\sim 0.1$ rad found currently, we furtherinvestigate whether the off-beam GRBs (in the uniform jet model) or theoff-axis events (in the structured jet model) can significantly enhance theGRB/GW association or not. For the former the enhancement is at most moderate,while for the latter the enhancement can be much greater and a high GRB/GWassociation probability of $\sim 10\%$ is possible. We also show that the dataof GRB 160821B may contain a macronova/kilonova emission component with atemperature of $\sim 3100$ K at $\sim 3.6$ days after the burst and more dataare needed to ultimately clarify.

Short GRBs: opening angles, local neutron star merger rate and off-axis events for GRB/GW association