Can particular stretching exercises help treat sciatica?

Maybe. It is conventional to use exercises for the treatment of sciatica and a variety of back conditions. These exercises should be specifically tailored for your particular needs.
Core Exercises. Typically core strengthening exercises are the best to help relieve pressure and forces in the spine. I would try and google some or look them up on youtube to see descriptions. If you can do them yourself, wonderful otherwise ask your doctor to get a referral to a good physical therapist for further assistance.
Yes. Specifically if some of your pain is coming from a common condition called piriformis syndrome.