Ohhh sciatica at 12 weeks is quite early lah, but it does happen to some mums especially if you're carrying differently this time or your ligaments are relaxing faster in the second pregnancy, a few of us in my mum group actually complained about lower back pain starting earlier the second time around! Best to mention it to your doctor or gynae just to rule out anything else and get some proper guidance on safe stretches or physio, because pregnancy sciatica can get quite uncomfortable if you don't manage it early. Hang in there, the positioning and your baby's growth will shift things around, so it might ease up as you progress!