With the legs crossed, that might call for two simulations, or at least two steps in one simulation. First simulation or step would start with scaled down legs, cross them in a standing position, and end with normal scale. Second simulation or step would bend the legs to seated position. The potential problem of doing it all at once is that the skirt could get pinched between the thighs while they cross, and I'd want to be careful to avoid that. There is another potential problem with the skirt getting pinched between thighs and pelvis in seated position, but that's easier to deal with if addressing it independently of the potential leg-crossing pinch. (Plus, a potential pinch between thighs and pelvis may not even matter in the final render.)

Here's why pinches are a problem. The simulation is attempting to keep the cloth at a distance (determined by collision offset) from each of the collision objects (each body part, and the furniture if you are including it). If the cloth gets sandwiched between two collision objects (i.e. left thigh and right thigh), and the collision objects are too close to each other to allow the cloth to be at the collision offset distance from each, then it becomes impossible for the simulation to do what it's supposed to do and frequently breaks the simulation.