The objective is the junction zone of the multilateral fishbone wellbores planned at the Yamalo-Nenets Autonomous District oilfield. Reservoir rock stability around the junction area is estimated with the help of the full coupled 3D poroelastic modeling. In order to determine the optimal fishbone location, the different cases with the various sidetrack inclination, parent well orientation along the maximal and minimal horizontal stresses with the condition of sidetracking in the rock with standard and reduced strength were simulated. The permissible depression and repression values were estimated for each fishbone configuration. For the cases with unstable junction the safe pressure drop and overbalance for drilling regime were determined. It was shown that in the case when mudcake is formed on the wellbores wall the pressure drop can be essentially decreased with fishbone stability retention.