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Add some bones to control the movement of the breasts, it is necessary that the bones for the control are above the bones of the breasts, and parent the bone of the upper body 2 [上半身 2],

then parent each chest bone to the added bones, then to the new bones add a rigid body of the type "bone tracking"

In the "Join" tab, join the rigid body of each chest with the rigid body of each bone of the chest control, that's all ...