My sister is experiencing some weird symptoms in her elbow. could she be out of shape for tennis?

Probably: Without knowing the specifics of the symptoms, this is hard to answer. Obviously, tennis can stress the arm in ways that normal activities of living do not. Therefore, it can cause unique symptoms that the arm can adapt to if given enough time and if not subjected to too much volume or intensity. If she fails to improve, she should see an orthopedic surgeon.

Hard to figure: Without an exam an exact diagnosis for weird symptoms is hard to ascertain. If symptoms are mild and resolve with ice and rest, it's probably just overuse. If the symptoms persist or worsen an exam is necessary, she may require a local injection or physical therapy to help resolve the problem.