Teacher Jeremy Forrest plans to marry the schoolgirl he abducted, and her father said he would be "proud" to walk her down the aisle.
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.
The teenager's father told the newspaper: "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.
"I'd like to visit him in prison, shake his hand and say, 'Thanks for looking after my daughter'. I would be nice to him."
Ms Hanspaul said her brother had been "vulnerable" at the time of his offences, and described 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 told the newspaper: "Jeremy mouthed to me, 'look after her, comfort her', because she was really upset. So I just turned around and told her it was Ok and put an arm around her.
"I know that she's the girl that my brother's in love with. It was hard that she was so upset and that was upsetting him.
So I suppose I instantly felt protective. I didn't want to see her sitting there crying.
"If they're going to carry on and be in a relationship, we would welcome her into our family and offer any support she was to ask for. We are concentrating on getting him through this."
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.
The teenager has written a letter to Forrest's parents, in which she apologises for "any upset I have caused" and makes clear that her feelings for him cannot be changed, the Mail On Sunday said.
In a five-page note, she says: "I can't emphasise enough how everything I feel towards Jeremy is - more than anything - serious... he is wonderful.
"I understand that when Jeremy comes home there will be things to sort out and the main concern will be making sure that Jeremy is able to cope and adjust to living in the outside world again.
"Obviously I now and will continue in the future to stand by him. I do believe that some things are worth fighting for.
For now, though, I'm going to concentrate on working hard to get the future both me and Jeremy want and I'm
preparing for college.
"Jeremy will be pleased to know I have been accepted on an A-level maths course - maybe he would find it comical as maths was my worst subject at school."
Forrest's parents last night told the Mail On Sunday that their son "knows he's been stupid", and is "paying for a mistake".
They said that depression had caused him to run away with the teenager. His father, Jim, said: "As parents, there have been tough moments but family is very important to us and we're behind Jeremy.
"He has been very stupid - obviously running away with a pupil is not on - but the Jeremy who did that is not the one I know."