The so called `Virtual Torsion Sensor' (VTS) has been introduced in pervious works for flexible joint robots without sensing of the joint output states, i.e. link position and velocity. Since VTS is incorporated into the feedback control loop, so as to improve the links' positioning accuracy, the related stability is crucial for the overall control design and robust operation of VTS. In this paper, we analyze the stability of including VTS into the feedback loop while assuming the predicted joint torsion is gained by the proportional term of the underlying motor position feedback control. We start our consideration by an ideal case of the linear joint stiffness, first assuming the measured joint torsion and after that using VTS instead of the measurement. Then, we move to the case of nonlinear joint stiffness with a cubic polynomial term, which captures the well-known hardening effect in transmission of the flexible robotic joints. The sector condition is formulated and Popov's criterion is applied to show the absolute stability for bounded range of the relative joint torsions.