The girl, who has written to the Scots-born teacher’s parents to declare her love for their son, intends to visit him in prison.
Forrest, 30, who was born in Aberdeenshire, was jailed at Lewes Crown Court for five-and-a-half years on Friday for child abduction and five charges of sexual activity with a child, after he fled to France with the girl when she was 15.
But Forrest’s sister, Carrie Hanspaul, 33, said that during a prison visit he told his parents: “I love her and we want to get married.”
The girl has already spoken of her determination to keep her relationship with Forrest going, and her father told a Sunday newspaper that he condoned her plans to see him in prison.
He said: “My daughter had a very unsettled family life when she was growing up. It would be a dream for her to settle down and marry the man she loves. She would want to do it all properly. I’d be proud to walk my daughter down the aisle with the man she loves. I honestly believe he loves my daughter.”
The teenager has written a letter to Forrest’s parents, in which she apologises for “any upset I have caused”.