Thanks for telling me that it's still confusing. I can see how it is. I'll rewrite the chapter so it's less confusing for everyone, but for now I'll just answer it here.

Salazar hadn't known that Syrene was pregnant with Syra at the time. However, he happened to be in the Quill Room at the time of Syra's birth and noticed the quill write down "Syra Slytherin." It was entirely a coincidence he was there. He immediately casted a spell on it to make her name appear centuries later. How the bloodline can't be traced (unless someone really digs) is because according to the record in the Quill Room, Syra was born in the late 1800s, which in truth is just when she woke up because Syra stopped giving her the potion. So when Syra woke up, Syrene allowed her to age naturally and raised her as if nothing had happened.

Syra did actually give birth to Aurora in 1961, so Aurora is actually 30 right now. Aurora did notice things about her mother while growing up, but she has kind of a rocky relationship with her mother because of this (mostly because she felt like her mother was always keeping secrets from her) so she hasn't seen Syra for a long time. I know we haven't seen Syra yet so that wouldn't have been clear, but Syra's coming up really soon, which should make things more clear. You are right with your assumption. Syra doesn't age. However, magic does wonders. :)

I realize that this really doesn't sound believable, but magic just seems like it can do anything. :) Thanks for the review and asking me. :)