Our Femur bones nestle into the hip socket & aid flexion, extension & rotation of the hip joint.
When we think about hip internal & external rotation, we consider what is happening at the hip joint & not what is happening at the foot. For internal rotation, the inner thighs roll inward & towards the back, you are turning your femur (thigh bone) inward towards your pelvis. For external hip rotation, the inner thighs are rolled outward & to the front.
You may have to work through some muscle tension in your adductor muscles, the internal rotators, & deeper tissues.
But even after doing this, what will allow us to safely access this particular posture is the shape of the two bones that make up our hip joint: the femur & the pelvis, & their functionality for external rotation that is required for the Lotus pose.