Hi Lassia!
I've never heard that name either, but I think your guess (it might be related to
Sebastian) seems to be a good one.
In Sweden there are currently just 15 women with that name and noone at all in Norway and Denmark, so it seems to come from another part of the world.
A Russian form of
Sebastian is
Sevastian, and they are usually a bit closer to the Greek pronunciation... Maybe the Greek experts here can help?
Pavlos??
Terveisin
Satu