Becoming Jane

Jane Austen is the younger daughter of Reverend George Austen and his wife and has yet to find a suitable husband. She aspires to be a writer, to the dismay of her mother and proud delight of her father.
Thomas Lefroy is a promising lawyer with a bad reputation, which he describes as "typical" for people in the profession. Tom makes a terrible first impression upon meeting Jane, when he nearly falls asleep while she gives a reading of her work for the company. Overhearing his subsequent criticism, Jane cannot stand the arrogant Irishman. Meanwhile, she turns down the affections of other men, including Mr. Wisley, the nephew and heir of the wealthy Lady Gresham. Wisley proposes but Jane ultimately rejects him due to lack of affection. The mischievous Tom encounters Jane again; they argue but increasingly take interest in each other and Tom demonstrates that he takes Jane's literary aspirations seriously. In time they fall in love.
Tom, Jane, her brother Henry and Jane's rich widowed cousin, Eliza, Comtesse de Feullide, conspire to receive an invitation from Tom's uncle and benefactor, the Lord Chief Judge Langlois of London, for the rich "Madame La Comtesse" and her friends. This visit is meant to be a short break in their journey to see Jane's brother, Edward. This would allow Judge Langlois to get to know Jane before and give a blessing for their marriage. Full of hope, Jane cannot sleep during the night at the Judge's place. In a flow of inspiration, she then begins the writing of First Impressions, the manuscript that will become Pride and Prejudice.
However, Judge Langlois receives a letter informing him of the genteel poverty of Jane's family and he refuses to give Tom his blessing, declaring that he would wish Tom to be the whoremonger he had been rather than allow him to live in poverty because of a bad marriage. Tom tells Jane that he cannot marry her and she is crushed, not knowing that Tom has a legitimate reason; his family depends on him financially.
Jane returns home and soon learns that Tom has become engaged to someone else at the arrangement of his family. Jane accepts the marriage proposal of Mr. Wisley, whom she had earlier declined. Later, Tom realises he cannot live without Jane, and returns, asking Jane to run away with him, for "what value will there be in life, if we are not together?" Jane agrees, and they leave, with only Jane's sister Cassandra knowing they plan to marry in secret.
On the way, Jane stumbles upon a letter from Tom's mother, and realises his situation: he sends money he receives from his uncle back to his parents and siblings, and his family cannot survive without it. She tells Tom that they cannot elope, not with so many people depending upon him. He insists that he and Jane must marry and tells her he will earn money, but Jane tells him that it will not be enough; he will never be able to make enough money to support his dependents with a High Court judge (his uncle) as an enemy and with a penniless wife. Distraught, Tom asks her if she loves him, and she replies, "Yes, but if our love destroys your family, then it will destroy itself, in a long, slow degradation of guilt and regret and blame." As she leaves, Jane catches a last glimpse of Tom through the carriage window as he briefly follows, the horses outpacing him.
Twenty years later, Jane, now a successful author and by choice unmarried, sees Tom during a gathering. Henry, now married to Eliza, brings Tom to her. Tom introduces his eldest daughter, who admires Jane's novels. As she asks Jane to read aloud, he remonstrates with her by her name, also Jane.[clarification needed] Astonished that he named his eldest after her, Jane agrees to read. The last scene shows Tom's daughter sitting by Jane as she reads aloud from Pride and Prejudice, while Tom watches Jane affectionately. As she concludes, their eyes meet, and Tom joins the rest of the company in honouring Jane and her work with applause.

Question: Who is the only person that knows Jane and Tom plan to marry in secret?
Answer: Cassandra