Just to make sure we are on the same page, I am asserting that Conan could be using Masumi's prior testimony that Mom, Masumi, Shuuichi all look alike to reach the conclusion that the girl is the mother. In order to reach that conclusion, Conan needed to know that Masumi and Shuuichi were siblings for sure, and he got confirmation just last chapter, thus making this line of thought relevant. While Sera's parentage seems totally obvious to us viewers, if you check the list of clues that Sera was Akai's sister, most of them were things Conan was not aware of: the name clue, Andre Camel and James Black finding her familiar, Sera chasing Scar Akai, sleeptalking on the mystery train, etc. I don't think hiding Conan's conclusion from the reader is pointless because if Conan came out and said what he thought at this stage, it would ruin the mystery girl's identity.
That said, Haibara is also a good candidate because it would back the popular sisters theory.