how to fix the weight of hanfu

Hi. In-game, most of the time, sims stand with arms down. In MD, the avatar stands in a t-pose instead. When a garment with loose sleeves simulates in a t-pose, they look wrong in-game.
So, clothing like this must be made with arms down
You can try the following, but it most likely will not give you good results:
- separate arms from the reference;
- separate sleeves from the garment;
- transfer all data from arms to sleeves;
- join the sleeves back with the rest of the garment, remove doubles, check in-game. You will understand what I mean better once you see it.

The correct way of fixing this issue would be:
- watch this video first to try to understand what is being done and why ( there are subtitles );
- clone a lingerie set, export the blend, open it, select the head, lingerie set and feet, export them as .obj with Selection only enabled, name it T pose;

- then unhide the rig, change the arm position by a specific value as shown in the video. Export the same body parts again, name it A pose;
- go back to MD, freeze all patterns beside sleeves;
- delete the avatar, import T-pose, then import A-pose as morph target;
- you will see the avatar and sleeves change their pose;
- select only the sleeves and export them;
- import these sleeves into the blend file with the lingerie set, apply the armature modifier, transfer all data from the lingerie set to your sleeves;
- modify the pose again as shown in the video, apply;
- save the blend and append these sleeves into the blend with the hanfu. Delete hanfu's original sleeves and replace with these ones

Before bringing the arms up again, you need to remove doubles. This way this gap will be closed and the mesh wont rip
That said, I would recommend to just re-simulate this specific item instead of doing all of that. The way it is shaped currently will give you problems, no matter what.
This piece of fabric is right in front of the arm. It will either clip or move in a way that does not make sense from a point of realism. It is the same situation as the collar on your white coat, the arm will either clip into it or pull it.
It is best to just tug on this piece a bit in MD, so that it is more similar to the other side ( does not need to be identical, just less in front of the arm)


As for the sleeves, for the most part they are pretty tight, so there is no need in simulating this garment with arms down. You just need to make sleeves tighter in the armpit area and it all will work out

Hanfu:
- uv_1 is transferred with wrong setting, should always be Nearest face interpolated;
- after editing weights, always use Clean with All groups and Limit total. This is why spikes happen;

- weights on the arms look jagged, make sure to transfer them with Nearest face interpolated as well. Also, you should delete hands from a reference because sleeves get data from them. You would probably need to manually edit elbow bones, but fix other issues first;
- your mesh is affected by foot and skirt bones, meaning the 60 bones limit is most likely exceeded (I counted, there are 61 of them). This leads to distortions in random locations


Acc:
- uvs must not overlap. The top and the bottom of the accessory is lit and obscured differently, right? But their uvs share the same location, meaning both sides will be baked in the same place;
- it should be vertex painted with 007F00;
- also, for the future, never transform anything in object mode, it can cause problems. If you need to move, scale and rotate something, do this in object mode only;
- try to not go past 10k. There is simply no need for a small accessory like this to have higher polycount than maxis's most complex meshes, it accomplishes nothing