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I mostly think the unknown child is Masumi Sera's mother who is also just like conan and haibara a victim of the drug; apotoxin 4869. She also seemed quiet sharp-witted and seems to also act more discipline towards Sera judging by the way of her speech and attitude(a little similar to Haibara) which could only reckon more she is of an older age and the mother of Sera and akai etc.

Bourbon may be the brother of Akai and Sera The identity of Akai and Sera's brother may strongly be relating to of whom is currently disguised as the black organisation member, Tooru Amuro or codename 'Bourbon'. This may seem as an ignorant assumption and a quiet myopic fact however, we already know that the middle brother had more of his fathers looks passed down than his mothers looks which were passed down to sera and shuichi. Bourbon also only appears later in the series after Akai Shuichi's faked death which could be a result of bourbon entering the black organisation for the sake of finding and perceiving the truth of his brothers death or to search further for his brother, like he has done many times when disguising as Akai. We also know that bourbon is known to bear a grudge against akai which could also be another hint faked by Bourbon in order to get closer to akai and gain the organisations trust. It is also mentioned in one of the mangas that Tooru Amuro could be with the police. This is my theory of who the brother is.