In The Chrysalids, how do the births given by David's mom and aunt explain how it is a patriarchal society?

David’s mother completely supports his father in rejecting babies who do not conform to the True Image. 

A patriarchal society is one dominated by men, where women are subservient.  Waknuk is a very strictly religious society, dominated by David’s father.  The True Image is supposedly a representation of what all people, animals, and plants should look like.  Anyone or anything that does not conform to the True Image is cast out. 

David describes the...

David’s mother completely supports his father in rejecting babies who do not conform to the True Image. 

A patriarchal society is one dominated by men, where women are subservient.  Waknuk is a very strictly religious society, dominated by David’s father.  The True Image is supposedly a representation of what all people, animals, and plants should look like.  Anyone or anything that does not conform to the True Image is cast out. 

David describes the birth of his little sister, Petra.  The birth of the baby is very odd. No one will even acknowledge there is a baby until it has been certified.  It is as if it does not exist. 

One did not dare to announce a birth until the child had been officially examined and approved; and the longer the formal announcement was delayed, the more time the malicious had to invent reasons for the delay. A man of standing looked to having the certificate granted at the earliest possible moment. (Ch. 7) 

When David’s mother’s sister brings a baby by that does not have its Normalcy Certificate, she refuses to support her sister.  David’s aunt is horrified, because she has not been able to have a baby that can be certified as normal.  We are never told what is wrong with the babies.  Whatever was wrong with this one was just a “little thing.”  She asks to borrow Petra in order to get the certificate.  This horrifies David’s mother. 

'In all my life I have never heard anything so outrageous. To come here suggesting that I should enter into an immoral, a criminal conspiracy to ... I think you must be mad, Harriet. To think that I should lend —' She broke off at the sound of my father's heavy step in the passage. (Ch. 7) 

She asks her husband to send her sister away.  She knows that he would never approve, and she doesn’t want to do anything to upset him.  The baby’s father rejected it too.  The inspector is also a man.  The men are the ones who determine if the babies can be kept or not, and there is nothing the women can do about it.