Unbeknownst to Willie, his birth father was not the elderly Earl of Ellesmere at all, but Jamie Fraser.
While Brianna knows who William is, William does not yet know that Jamie is his father and that Brianna is his sister. He doesn't discover the connection until late in book seven, when he finds himself in Jamie's company and finally sees the resemblance between them.
Season Two
With regret, they report that they've lost Willie – to marriage. Claire is pleased to hear that Willie has married and moved to the colonies, and suggests that marriage wouldn't do any of them harm.
William loses both his mother Geneva, who dies during childbirth, and his supposed father the Earl (killed by Jamie), shortly after his birth, from there he ends up in the care of Jamie's friend and close confidant Lord John Gray - who had married Geneva's sister Isobel.
The short answer to if Jamie will ever see his son again is yes, but the long answer is that it is going to be a few years before that happens.
Grey falls in love with Jamie, who rejects his advances. Still, Grey cannot dismiss his feelings.
Claire asks Lord John if he believes she (and Brianna and Roger) are from the future, to which he says that he doesn't, but that he will behave for all appearances as though he does.
A Breath of Snow and Ashes
William meets Brianna MacKenzie, not knowing that she is in fact his half-sister but feeling as though she looks familiar. They spend some little time together before the MacKenzies leave and William returns to his regiment.
Let's learn about the disease that robbed Jocasta Cameron of her eyesight: Glaucoma. Yep, that's the one. Glaucoma (glaw-koh-muh), is a word derived from the Greek glaukommatos meaning “gray-eyed.”
until he was forced to make one of the most heartbreakingly difficult decisions of his whole life. Jamie had to leave his son behind, with no hope of ever seeing him again. Admittedly, Jamie's son Willie isn't exactly a character who has been around since the beginning of the series.
Claire didn't know anything about Jamie's son. Lord John Grey shows her a miniature, explaining Jamie has a copy of it (and that's what she saw in the previous chapter), and tells Claire the story of when Jamie asked him to care for Willie.
The underlying reason for John Dutton's dislike of Jamie is their differing personality and views on life. It is why they often fight, argue, and even work against each other in various stages of their life. Of course, it's also possible that John dislikes Jamie because he's adopted and not a true-blooded Dutton.
Sadly, William's birth resulted in the death of his mother Geneva and as Jamie was unable to step up given his position at the house, Lord John Grey agreed to raise William as his own.
(Remember, Jamie spent 7 years living in a cave and about 4 years in prison before arriving at Helwater and meeting Geneva.) So really, Brianna is about 12 years older than William. I believe Willie was 6 when Jamie left Helwater to return to Scotland.
Jenny meets Brianna Randall Fraser for the first time at Lallybroch, when the girl travels into the 18th century to search for her parents. Jenny is delighted at seeing that Jamie fathered a daughter as they had never expected him to have children, and marvels at Brianna likeness to him.
When Brianna travels to 18th-century Scotland, she marries Roger MacKenzie. They have a son, Jeremiah "Jemmy" Fraser MacKenzie, and a daughter, Amanda MacKenzie.
Father seems to formal, and she can't call him daddy, Frank was her daddy and to call him this would seem a betrayal, and she certainly couldn't call him Jamie! True to form, Jamie sees her discomfort and interprets it correctly, and tries to help alleviate her distress. “You can… call me Da, ” he said.
In the third series, Jamie says to Kayce: "I need some advice, brother... If I can still call you that." Kayce then replies: "Until the day you die, you better never call me anything else." This shows Kayce does know about Jamie not being his biological sibling but is accepting of him anyway.
While she is scared that it may damage her relationship with Rip, Beth does tell him about her being sterile. In response, Rip assures her that his feelings toward her won't change, even if they can't have biological children.
His descendants–a few of whom he meets now and then in his travels–have the blue light about them, too; in large degree or small, depending on their talents. So he knows Claire, when he sees her, as one of his great-great, etc. grand-daughters.
In September 1764, Lord John told Jamie he intended to marry Isobel Dunsany and become William's stepfather. Jamie offered his body to Lord John, who declined. Jamie kissed him.
But when the rubber meets the road - Jamie realizes that it's difficult to keep his promises. Because the fact that it was Lord John that Claire turned to - that somehow cuts Jamie deep. Clearly he still loves Claire, understands what she did, and forgives her for it - but here, in this passage, he admits he is human.