Hahaha. I'm pretty sure she does know who Conan is. Actually she's known from the beginning
I mean that she know he is a "boy from the beach" but I think she don't remember/know that he name Shinichi.
But I may be wrong ;)
so she probably think Conan and Shinichi are a two person
*different person
I think sera knows he is shinichi because she also knows ran. And sera's sister have also shrunk
I think the little girl who's always been with Sera is her mother who has shrunk.
Idk but I think that Sera'll find the true identity Conan soon ٩(๑❛ᴗ❛๑)۶ Because she don't know that Conan is Shinichi..... At least I got that impression