Kiri's stepfather Peter Newnes, 53, revealed Alex had to carry the blow-up doll around with him in New York as part of a stag weekend forfeit.
He told MailOnline: "Alex is an educated high-flyer but where was his common sense? Kiri is supporting him but we are all gutted by what happened on his stag.
"His mother is gutted too, he's a very intelligent guy, he's no fool. But if you look at the photographs you can see he just didn't realise what he was doing."
Kiri is said to have forgiven her fiancé and the plush wedding, a year in the planning, is due to go on at their dream wedding venue near Hereford on Saturday.
Mr Newnes said the timing of the inflatable doll prank, two days before the 15th anniversary of 9/11, was "dreadful".
He added: "Kiri is upset but she is not making excuses for what he did. It was a ridiculous thing to do.
"We don't agree with what Alex has done but I can assure you that is not him at all.
"He's an educated, decent, hard-working young man. We've all been on stag weekends but he's made a mistake.
"I have spoken to Alex and he's obviously upset. I can't understand what he was thinking at the time.
"We are all gutted at what's happened but we are not going to hold it against the lad."
Former rugby player Alex was on his stag weekend along with his banker brother Tom, 29, and their father Clive, 66, a retired schoolteacher.
The group of men also took a series of light-hearted selfies before they were eventually confronted by a police officer and asked to leave the area.
When asked what they were doing Alex reportedly said: "It's just a bachelor party".
Families of some of the 2,977 victims of the attacks on the Twin Towers, which included 67 Britons, were left furious by the tasteless stunt.
Rosanne Hughes, 61, from the New Jersey 9/11 Memorial Foundation said she was appalled to hear about the stag party celebrating on Ground Zero.
She lost her husband Thomas F. Hughes Jr, 46, who was on the 107th floor of the World Trade Centre for a meeting when the plane hit.
She said: "I'm speechless. I don't even have any words for that - that's a disgrace.
"That is sacred ground, I lost my husband and I don't even know what to say - it's disgusting.
"If I came face to face with these people I would say 'shame on you. You have no clue to what those families have gone through and to be that disrespectful' - they should be fined. They shouldn't be allowed to get away with that."
While Michael Frazier, executive vice president of communications and marketing at 9/11 Memorial and Museum, said: 'This image is exceptionally disturbing and what is just as disturbing - if not more - is that these people find it appropriate to blatantly disrespect this place made sacred by the murder of 3000 innocent people.'
Professional dancer and choreographer Kiri first saw the pictures of her fiance while on her hen weekend with mum Liane at a spa hotel in West Wales.
Neighbours at the groom's £370,000 family home in the leafy village of Rogerstone, near Newport, South Wales, could not believe their eyes.
One said: 'Clive is a retired maths teacher, he's a lovely fellow, very gentle and well-mannered.
"I was more than a little surprised to see him in the photographs, it's not like him at all. But he's a distinctive looking chap, I recognised him straight away.
"Alex and Tom left home a long time ago but I know there's been a lot of excitement over the coming wedding."
While Clive went on the stag weekend to New York his wife Sue has been staying at the couple's holiday home in Spain.
She is flying back at the same time as her husband and two sons are returning from their ill-fated trip to the States.
Brothers Luke and Josh Adams, who grew up with the Jerrett brothers in Newport were also understood to be on the trip.
Their father, company director Andrew Adams, 54, declined to comment on the sex doll stag weekend fiasco.
Kiri, of Cardiff, trained at the Northern Ballet School and describes herself as an actor and dancer. She has also modelled for Littlewoods catalogues.
Kiri, who has danced on Cunard and Thomson cruise ships during her showbusiness career, was too upset to talk about her groom's sex doll blunder.
Her grandmother Penelope John said: "I'm very proud of my grandaughter and I'm proud of her husband-to-be too."
The groom could not be reached for comment.
