what am I doing wrong with my dress mesh?

Hi,


I've learned to mesh with blender the past months. And now I'm happy with my Maxis match mesh I've made. In Sims4Studio, it looks good, no problems. I've did the weighting, the vertex paint, because of a transparent part, I've followed the steps of the tutorial here about the transparent skirt. So, I can't see what's wrong. I've double checked the double vertices, and no clue for me. 

Because of what's wrong in the Sims, I guess it has to do with doubles or the weighting, but for as I can see, I did it all right. Is here someone who can look at my files to see what's wrong? And maybe he or she can tell me how to get rid of it? 



This is how it looks like in Sims4Studio


This is how it looks like in the Sims 4. It's the waist that doesn't look good.


Here is my Blender file
And here is the package file.

Thanks for so far.