I've made figures sit on chairs before and haven't had an issue.  Maybe it's how you are setting up your simulation?

Here is what I do:

In Pose Room....

1.  Add figure
2.  Add dress
3.  Add chair

4. Go to frame 15 (of 30 frames)  and add your figure's pose so that she's sitting in the chair
5. Go back to frame 1
6. Move to cloth room

In Cloth Room...

1.  Create new simulation

• 20 frames
• Cloth self collision

2.  Clothify the dress

• pick your figure
• pick whatever else the dress will come into contact with IE: Stone, ground
• If not into contact with head, hands or feet, check the appropriate boxes. if those parts are involved, leave them unchecked.

3. Save your file

4.  Run simulation

"It is good to see ourselves as others see us. Try as we may, we are never
able to know ourselves fully as we are, especially the evil side of us.
This we can do only if we are not angry with our critics but will take in good
heart whatever they might have to say." - Ghandi