It's possible. When patients were surveyed in a recent study most reported a traumatic event as the initial cause of the problem. Car accidents and work-related incidents topped the list. Non-work related events are reported but the specifics aren't really clear.
Being pulled by the arm can put traction on the nerves to the upper extremity and neck. Thoracic outlet syndrome (TOS) is not a traction injury, but rather, a compressive injury. The nerves and blood vessels leaving the neck and traveling down the arm get pushed up against the ribs or squeezed between layers of fibrous tissue. Previous injuries to the neck and shoulders could be contributing factors to these new symptoms.
In 20 percent of the people affected there is no known cause. The symptoms occur without warning and no accident or trauma is involved.
Your concern for your sister is understandable. This might be a good time to gently ask a few questions such as:
Have you been hit, pushed, pulled, or punched by anyone?
Do you feel safe in your current relationship?
Is anyone from a previous relationship making you feel unsafe now?
Be prepared to offer information about local battered women services in her area. If she answers 'yes' to any of these questions, she may be ready to seek safety before further injury occurs.