As a so-called deviant, Aunt Harriet's baby is effectively a non-person in this society. The child might as well not exist. So when Aunt Harriet drowns herself with her baby, the child is never mentioned by her family ever again. Above all, this is because Aunt Harriet and her deviant offspring were a source of shame to her family. They've always rigidly adhered to society's belief that any kind of physical abnormality is a sign of sin. And so the last thing that Aunt Harriet's folks want is to acknowledge the fact that they have a sinner as part of the family.
There's also an element of fear involved in their reaction. Their silence concerning the deceased baby is deafening. It speaks volumes about the fear that someone else in the family might have somehow "inherited" the sin that infected Aunt Harriet, which will cause them to give birth to a deviant child of their own.
It is in Chapter Seven where this incident is mentioned. Aunt Harriet comes to David's mother with her new child, having lost two of her children already because of their deviations. Finding out that her third babe too is a deviant is too much, and thus she goes in vain to her sister to ask for her help. The harsh reproof she receives reflects the Storm family's unthinking devotion to the creeds they profess and show their lack of humanity, even though David's mother herself has experienced losing children because of their deviations. Thus when it is that Aunt Harriet, now completely hopeless, leaves them, she drowns herself and her baby, not being able to bear another separation. The baby is not mentioned simply because, being a deviant, it is not regarded or considered as a human being. This of course foreshadows the way that Petra is a deviant, albeit a hidden one.