I agree with Chekov that there's not enough clue yet to discuss the child's identity further. Gosho only gave us 2 minor clues:
That child's definitely got the so-called Akai-family-eyes.
He/she uses polite and adult-like diction (at least when he/she's speaking to Masumi), which is also a contradictory behavior despite the fact that he/she has a body of a mere child.
I my self prefer the theory saying that the child is the mother of the Akai-siblings, though I find it rather uninteresting if compared to the theory saying that the child is the brother. Since Gosho's always use scientific explanation or at least quite a logical reasoning, if we based on the child's hair color (which is likely to be blonde, IF he/she doesn't get it dyed or is wearing a wig, and it is very unlikely for a character in Meitantei Conan to have dyed hair) I think the child is likely a parent of the Akai-siblings.
Shuuichi is a Japanese-American. If we assume that the Akai siblings were born from the same couple, one Japanese and one American, then the probability of one child to have a blonde hair is only 1 out of 4 because black hair gene wins against blonde. And as we can see both Masumi and Shuuichi have black hair. It is still possible that Masumi's second brother is a blonde though, but the probability is rather small.
Well, I am too curious about the child's identity and Gosho's only left us with minor clues. I feel so damned right now that I couldn't think of any other theory. And yeah I am not going any further until there's some more clue~