"I didn't want anyone to talk to me," said Harry, who was feeling more and more nettled.
"Well, that was a bit stupid of you," said Ginny angrily, "seeing as you don't know anyone but me who's been possessed by You-Know-Who, and I can tell you how it feels."
Harry Potter and the Order of the Phoenix
I don't quite get the meaning "seeing as you don't know anyone but me" in this context. Probably, the phrase "seeing as" confuses me. How should we understand it here?