Sweater sleeves deform in CAS

You've got two rigs in your blend and weirdly neither seems to contain an actual rig. Delete both rigs and all of the rig's body parts and export just the top. Open your EA mesh and delete everything except the rig then append your mesh into Blender. File > append > double-click your mesh > double-click object > select your mesh > append. If an extra rig and/or a bone_bone_shape gets appended delete them.

Your mesh has some problems. For starters, the topology is terrible, and bad topology affects how a mesh animates. If you have a newer version of MD you can retopologize your mesh within the software.



Written instructions are here. Other videos are here.

You're having trouble because of the weight paint. The sleeves are picking up weight paint from b__Spine1__ and b__Spine0__.

You are going to have trouble because the sleeves are very loose around the armpit. If you look at an EA mesh you'll see that the sleeves are close to the armpit and there isn't a lot of loose fabric, either. Loose sleeves can cause this issue, too.

Go back into MD and, in addition to fixing the topology, fix the sleeves so that they are somewhat snug around the armpit.

Start over with the uv_1, vertex paint, and weight paint.

If, after transferring the weights, you're having a lot of trouble with unwanted weight paint on the sleeves, separate the sleeves in your mesh with P, delete all of the weights, and separate the sleeves in the EA mesh then transfer the weights from the EA sleeves. Then re-join the sleeves with your mesh with Ctrl + J.