I am by no means an expert on sciatica. Saddles themselves should have no direct effect because of where the sciatic nerve is. You would need a particularly wide saddle to come close to here the nerve resides in the back of your leg:

https://www.spine-health.com/video/s...-anatomy-video

However, sciatica is primarily associated with lower back vertebrae. A good fit and a good saddle should allow you to sit with a lower back angle that is helpful for sciatica, rather than needing to assume a bend that will make things worse. I've seen claims that cycling can even help with sciatica. Physical therapists are getting pretty good with cycling related stuff - use your health insurance.

One thing people do miss in getting fit are insoles. Most foot numbness comes from the way cycling shoes and cycling collapse the arch. Propping the arch brings many people relief from sciatica-like numbness in their feet.