I don’t think it’s the skinning weight that is the problem.Double check if your bone rotation axis goes with Unreal’s bone setup.
Try exporting out one of Unreal’s mannequins to blender (export the skeletal mesh as fbx and import) and examine the bone orientations along the arm (left and right side).
You may have a situation where the orientation is wrong and the shoulder bone / bicep is being rotated on import.