How do you describe this particular way of sitting?
There is no single word to describe this in English - you have to use a phrase, such as:
She is sitting on the floor with her knees bent and legs to one side.
Based on a Google Image search, plenty of results show up for the term "side-sit" and "side-sitting." They all seem to depict this position quite well. To me, "side-sit" appears to be the best way to describe this pose.