Mr. Tanner, the third person narrator, is anxious to explain how there was a lawyer in the family.
Tanner and Kitty were both poor servants when they married, and Tanner was illiterate. It makes sense that he would be proud of his daughter, who grew up to be so important and educated.
Mr. Tanner was anxious to explain how it was that he had a lawyer in the family, so that when they all decided to sell up and quit New Zealand there had been someone they could absolutely trust with the legal business. (p. 362)
The story begins this way, demonstrating that it will be told in flashback. As a matter of fact, Mr. Tanner and his wife Kitty had many children, but their first birth was twin daughters. Mr. Tanner accidentally threw out one of them, thinking it was afterbirth. She is the one who went on to be a lawyer.
This opening sets the tone of the story, which is one of irony and humor. It also establishes that the story is going to be told in flashback, and that it is third person omniscient but Mr. Tanner is telling the story to others.