I don't believe so. The limits to each boss are drawn at certain points at in Darkroot it is at the magic door and at the top of the waterfall. I could be mistaken. You could always join the Forest Hunters Covenant if you want to pvp there.
I may have misread your question. I was assuming you meant the forest area.
As a phantom you can invade anywhere, pretty sure thats not what you're asking though, so, if you killed sif and not the moonlight butterfly no one can invade you in the sif forest or anywhere past the seal, but you can still be invaded in the forest section with the stone giants before the butterfly.