A $\rm (\bar{1}11)NiSi_2 //(1\bar{1}5)Si$ heterotwin interface has been observed by high resolution electron microscopy (HREM) along the common direction $\rm [\bar{1}\bar{1}0]NiSi_2 //[\bar{1}\bar{1}0]Si$ to identify the long period atomic structural units (ASU's). Coexistence of two different structural doMayns has been found with ASU's corresponding to two different cycles of five-, six-, and seven-atom rings. A dislocation/ledge (DL) has been particularly studied because its separates these two unusual cycles of ASU's. The elastic displacement field u of the atom columns around the DL has been calculated using both crystallographic and anisotropic elasticity properties of the crystals, assuming a two-dimensional u field. From a comparison between the positions of the atom columns as derived from elasticity calculations and as deduced from the HREM image, it is concluded that the DL is a mixed dislocation with the Burgers vector $\rm (-1/4)[111]NiSi_2 + (1/2)[1\bar{1}0]Si= (1/12)[515]Si$, a result also in agreement with the imperceptible ledge height associated with the DL core. Such a Burgers vector underlines the role of the Si matrix dislocations for producing such structural doMayns.