A numerical study was performed to investigate the effect of the area ratio at sister hole applications on film cooling effectiveness and flow structure. Two secondary coolant holes are placed on both sides of the main hole at a lateral distance of one main hole diameter. Four different area ratios (AR = 2–5) between the main hole and sister hole were studied. A single cylindrical hole was also studied for comparison and baseline. The structured mesh was generated and realizable k-ε turbulence model with scalable wall function was used in these simulations. Blowing ratios of 0.25, 0.50, 0.80, and 1 were simulated to evaluate the effect of sister holes in practical applications. The centerline cooling effectiveness and laterally averaged cooling effectiveness are studied as well as temperature contours and streamlines at two different streamwise locations (x/D = 1 and x/D = 3). The area ratio on sister hole application has an important effect on cooling effectiveness as the sister hole exit area affects the Anti- Counter Rotating Vortex intensity (A-CRVP vorticity) and size. The positive effect of the sister hole is more pronounced with the increasing blowing ratio. Lower are ratios such as AR = 2 and AR = 3, decrease the vorticity of the CRVP around %30 which directly affects the cooling effectiveness. The results of this study showed area ratio, as well as lateral distance between main hole and sister hole centers heavily affects the cooling effectiveness, the lower area ratios are more efficient for film cooling.