The girl, who has written to Forrest’s parents to declare her love for their son, also intends to visit him in prison.
Forrest, 30, 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, told the Mail On Sunday 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 The Sun Sunday that he condoned her plans to see him in prison, saying he wanted to as well.
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 and she’s besotted with him. She has made it clear she wants a future with him. Hopefully with good behaviour he may be freed on parole after a few years.
“If they have a future together that’s fine by me and they would have my blessing. I should hate him for what he’s done but I don’t — in fact, I’d like to meet him.
Ms Hanspaul said her brother had been “vulnerable” at the time of his offences, and described him as being immature and someone who “hadn’t fully grown up yet”, as if he was in his “early 20s rather than 30”.
She said she felt protective towards the girl, and comforted her in the moments after the jury returned the guilty verdict in court last week, the Mail on Sunday said.
Ms Hanspaul said she didn’t see the age difference and relationship between her brother and the girl as “predatory or grooming”, and that Forrest had told their father that he is happier now than he has been for a couple of years.
Forrest’s parents said their son “knows he’s been stupid”, and is “paying for a mistake”.
They said depression had caused him to run away with the girl.