There are three solutions that I can think of:-- Build the dress with hierarchy to allow seated poses as above.

• Include shins in the leg sections of the dress. Although you will still have to watch what happens to the hip.
• Use morphs to obtain the shape of the dress you require.

Or a combination of the above. Whichever way I doubt if its possible, in practical terms, to design a mesh that will be perfect in every pose.