If you read the chapter again, she didnt suddenly out of nowhere able to talk, but she gave and put all her effort and practiced talking,..
The only thing a bit quite different from her and other normal babies is that, she's a frkn adult inside a baby's body... but yh
Not trying to argue btw, just, uk, expressing my own opinion too, just like what u did..
From the outsider's pov it IS weird and abnormal but it is explained to us readers that she's an adult resurrected in baby form. Frankly, the protagonist should have thought that her progress is too special but I guess she never encountered a baby so as to make a comparison (she died young after all). Also, it is preposterous that the reaction of her family specifically the adults, is not to be scared, question, and "throw the whole child away" and instead, let it slide with saying she's a genius. That being said, it IS to show to us that the family is not the kind that will abandon their child just because she's being weird. Their continued support is to prove that they truly love her which is why our MC is striving hard to protect them.
If it were you and you would have thrown your own child away, then that speaks volume of what feelings you have with your own flesh and blood and thus, it would be questionable why your child (MC) would even strive to protect you since you abandoned her from the start.
(=・ω・=)

I was about to drop this manga, but then they had a time skip. A baby talking was getting out of hand, if I was them I would have threw the whole child away