Here is the translation of the panels courtesy Fujiwara
Note lots of people think the child is actually a boy, not a girl like Conan thinks. It would be fitting considering he was also mistaken about Sera.
Regarding the little mystery kid there seems to be two major lines of thought:
#1 The girl in the picture is Sera, and the kid is a relative. Sera didn't say she had any additional family members beyond her parents (her father is dead), herself, and her two brothers (872 pg 3) so the child must be related to someone in her family. The middle brother is old enough to have a kid of that apparent age. Another option would be that Sera's mom remarried after her first husband died, and had another child which Sera doesn't consider to be one of her siblings for some reason.
#2 The girl in the picture is Sera's mom (Who looks a lot like Sera!), and the child is Akai. The child is distinctly not smiling and its face is identical to Akai's. Sera's mom appears not to be chesty, but maybe she was a late bloomer? It makes sense in that Sera might be trying to hint to Conan her family's identity, especially after Conan was poking around the issue in 872. If the child is Akai, maybe he dyed his long blond hair black later? Do note that Char Aznable has long and blond hair with a very similar hairstyle to the child's. For picture comparisons, see the spoiler box below.
...