What is the force responsible for instantaneous spin change in sibling fermions upon re-orientation?
It’s not a force. It’s less than that. There is no re-orientation because the orientation previous to the observation is undetermined. This process is permitted by Relativity because no information travels instantaneously. The observer of the first particle has no control over which spin will be observed, so the process can not be used to send a message to the other particle, so the process is not an exchange of information, so it can occur faster than light if it likes.
