In a recent blog, I wrote about the general but oft ignored belief that cousins should not marry. Cousin marriage was fashionable in Jane Austen's time among the wealthy, but it also happened more than once in Jane's immediate family. Her brother Henry (top, by headline) married their cousin Eliza, and the son of brother Frank married the daughter of brother Charles. Cousin marriage also occurs in "Mansfield Park," when Fanny and Edmund are betrothed.

An even closer—and absolutely prohibited—degree of consanguinity is that of brother and sister. Sibling marriage being an incestuous taboo the world over, one would not expect such a thing ever to enter the environs of Austenia. Yet tradition brought it to Jane’s doorstep, for the law not only forbade marriage between blood siblings but also between brothers and sisters by marriage.

Therefore, the marriage of Jane’s brother Charles to Harriet Palmer after the death of his first wife was “voidable” because Harriet was Fanny’s sister. As explained in Martha Bailey’s article in “The Marriage Law of Jane Austen’s World” (Persuasions, Winter 2015), this sisterhood created a prohibition by “affinity” (marriage) as strong as one by blood. The logic was: Because Fanny and Harriet were related by blood, and because husband and wife became one flesh upon consummation, then Charles would also be related to Harriet by blood. This thinking applied equally for a woman who married the brother of her dead husband.

“Voidable” in Charles’ case did not necessarily mean “voided.” Someone—most likely a relative seeking to grab an inheritance—would have to sue to have the marriage voided and any children declared illegitimate. Charles never had enough money for anyone to bother trying to disinherit his four children by Harriet.

To resolve the ambiguity about people marrying the sibling of a deceased spouse, the 1835 Marriage Act validated all previous such marriages but voided any going forward. To evade this prohibition in still another Austen situation, Jane’s niece Louisa Knight went to Denmark in 1847 to marry Lord George Hill, who had been married to Louisa’s now deceased sister Cassandra. Such dodges continued until the affinity laws were removed in 1907.

This concept of “affinity” as a barrier to marriage brings us to the most difficult “brother and sister” pair in Austen, Mr. Knightley and Emma. Their “affinity” is not a technical one under the law but one created by proximity and time. In all but blood, Mr. Knightley functions as Emma’s older brother. He’s the good-natured scold who tries to keep a bright but undisciplined young woman on the straight and narrow and who also seeks to protect her in a fraternal way.

Modern courts have struggled with psychological affinities even when no biological issue exists. Adoptive parents have wanted to marry adoptive children, for instance. Did the love grow naturally as happens in any other relationship, or did the parent use a position of authority to groom the child inappropriately? Courts have looked at the amount of the disparity in age or the length of the relationship to try to determine the right course. Mr. Knightley is seventeen years older than Emma—and has known her since birth!

This is not to imply there is anything immoral in the Emma-Knightley relationship. Readers quickly recognize that they are the only two people worthy of the other and are intrigued at how they will surmount the barriers between them—primarily those Emma herself creates with her matchmaking exercises. This almost subliminal conflict is, however, one of many ways that Austen develops deep psychological issues in the courtship genre where other authors never reach beyond the superficial.

Their relationship begins to change as each (incorrectly) foresees the loss of the other—Emma to Frank Churchill and Mr. Knightley to Jane Fairfax. Feelings break through at the Westons’ ball.

First, Emma notices his physique: “so young as he looked! … His tall, firm, upright figure, among the bulky forms and stooping shoulders of the elderly men.” Then there is his gallant rescue of Harriet after Elton’s snub. “Never had [Emma] been more surprized, seldom more delighted, than at that instant.” Finally, there is their brief but serious tête-à-tête. The well-known and memorable exchange that follows ends up redefining their roles. Note that it is Emma who nudges the two forward.

“Whom are you going to dance with?” asked Mr. Knightley. She hesitated a moment, and then replied, “With you, if you will ask me.”

“Will you?” said he, offering his hand.“Indeed I will. You have shewn that you can dance, and you know we are not really so much brother and sister as to make it at all improper.”

“Brother and sister! no, indeed.”

The sense of this scene turns on how the key phrases are handled. In the 1972 BBC series starring Doran Godwin and John Carson, the brother-sister comments are treated as so much banter. In the 1998 movie, Gwyneth Paltrow also tosses her line out merrily. What follows, however, makes all the difference. Jeremy Northam first responds with a laughing “Brother and sister! … ”

Then, as she moves out of earshot, he adds meaningfully, “No, indeed.”

He understands the tectonic shift that has occurred.

When the book opens, Emma, in addition to being handsome, clever, and rich, has lived “nearly” twenty-one years. In the months that pass covering the events of the novel, she comes of age. This is not likely to be a casual choice for Austen. Twenty-one is the age of consent. This is Austen’s signal that the heroine is no longer in the junior position but is fully capable of owning this momentous swing from younger sister to adult wife.