Flexible robots have been widely used in minimally-invasive surgery because of their dexterity and accuracy. However, the torsion of end-effector under bending state usually produces unnecessary concomitant motion of the robot arm. This letter proposes a novel design of in-situ torsionally-steerable tendon-driven flexible robots for otolaryngology surgery. This kind of robots have three motion modes: pure torsion, pure bending and fusion modes. Among them, the fusion mode provides a novel in-situ torsional motion which can maintain the position and approach vector of the end-effector during the operation. A constant curvature-based kinematic model has been presented to handle different motion modes. Finite element analysis (FEA), numerical simulations and experiments have been conducted to test the robot design and the proposed model. The FEA shows that the maximum torsional deformations are 0.114 mm and 0.908 mm for straight and maximum bending configurations, respectively. The position, rotation and direction errors of the prototype are 1.53 mm, 2.08$^\circ$ and 2.71$^\circ$, respectively. The results indicate that the proposed flexible surgical robots are feasible for in-situ torsional motion.