Timeline:
Prologue: Chapter 26 - 'The Second Task'.
Begins: During Fourth Year.
Continues: After Deathly Hallows, possible flashbacks.

Alterations to Canon:
None, beyond fanon tropes.

Pairings:
Harry/Fleur.
Harry/Gabrielle.
Harry/Fleur/Gabrielle.

So yes, femmeslash incest.

Summary:
During the Second Task of the Triwizard Tournament it's a little unrealistic that Harry, who I can't see having been taken to the pool or for swimming lessons by the Dursleys, so quickly adapting to swimming in the Black Lake, Gillyweed or no. Instead it takes him a couple of minutes to figure out his strokes and get the hang of it, which delays his journey slightly.

As a result he hasn't passed Fleur yet and when he comes upon her in the middle of a field of kelp/seaweed being beset by a score of Grindylows he pauses only for a few moments to ogle the billowing curtain of silky blonde hair swirling around the pale, swimsuit clad curvaceous figure of the French Champion before diving into the fray and assisting Fleur in a fighting retreat.

Together Fleur and Harry complete the task rescuing their hostages and Harry once again receives a firm kiss from Fleur once they are back at the starting dock. In the subsequent weeks Harry and Fleur spend more time together, go to Hogsmeade, study together for the Third Task and become good friends. Fleur's interest and desires continue to grow and is both aware and thankful of Harry's reticence to act on his attraction towards her, because she's tempted as it is and at 14 he wouldn't even be legal in her homeland (consent is 15 in France).

During the Third Task, due to different scores in the Second Task and entry times, Imperius'd Krum finds Cedric first and it is Harry and Fleur who go on to take the Cup together. Thanks to Fleur's allure impeding Wormtail Harry is able to knock her to the ground so that the Killing Curses misses her before they are both stunned. Tied to a headstone in the graveyard she witnesses the Dark Lord's resurrection, the following duel and then escapes with Harry back to Hogwarts.

Fleur consequently joins the Order from its inception and is present at Grimmauld Place during the summer and later at Christmas, when Arthur is hospitalised after encountering Nagini. Learning of Harry's difficultly at grasping Occlumency she volunteers to help him learn it by focusing varying degrees of her allure on Harry as he attempts to clear his mind of the effects. Harry makes a lot of headway in that department, but Fleur loses all restraint in the other department and the two share their first time.

How much canon is followed or deviated from between then and the end of Book Seven is not particularly relevent except for what the author wants to include in what follows next.

If we accept Gabrielle is say 10 years old during HP&GoF then 7 years later, at least 5 years after the Battle of Hogwarts (or however the author writes Voldemort off) then Gabrielle is 16 or 17 years of age. Adjust it however you want to write it, but I really think making her 'under-age' for what follows would defeat the point.

Something has gone wrong with Gabrielle's Veela heritage.

Veela usually remain pre-pubescent later than humans, a defence mechanism to make them sexually immature and safe from the consequences of Allure-enflamed men until their magic and skill are developed enough to defend themselves. However they usually rapidly mature just before or after their 15th birthday, essentially becoming fully mature physiologically in as little as a 3 to 6 month period.

Gabrielle is just shy of her 17th birthday and to her horror, distress and depressed despondency has barely a curve on her body, a chest almost completely flat and has the looks of a 13 year-old. In desperation Appoline, her mother, summons representatives of France's largest Veela coven to evaluate her condition and they explain that it is an uncommon disorder/ailment where a Veela's nature doesn't awaken/activate itself - a sort of stunted growth. Sometimes it a Veela remains un-awaken for their whole life but there have been instances of Veela afflicted the same way being able to trigger the maturation by engaging in carnal acts...

This is no reassurance to Gabrielle who has, in her dark moods and despair, already arrived at the conclusion that no man will ever look upon her with desire and so there's no chance for her anyway.

Fleur's heart goes out to her sister, her plight is very distressing to the older sibling. Later that evening as she is 'distracting herself' Fleur collapses on Harry and murmurs that she is so thankful for having Harry in her life and just wishes that her sister had found someone like him. Harry assures her that somewhere out there is a bloke who will see Gabrielle for who she is and fall for her.

Fleur is less sure. As a pre-teen Gabrielle used to be engaging, curious, mischievous and energetic as she charmed everyone she met, including Harry. Her outgoing attitude and appetite for adventure had defined her, dragging friends and relatives off on adventures in the dunes around the Delacour holiday beach house, down alleyways and thoroughfares in Muggle Paris, around the extensive grounds of the Delacour estates... but when her Awakening didn't materialise after her 15th birthday she spiraled down into little more than a sad, broken and shut-in. How will anyone see her for who she really is?

Harry assures her that Fleur does and so does he and therefore Gabrielle will never be alone or forgotten, like Fleur and Gabrielle fears. Over the next few days as Fleur witnesses Harry try his best to cheer Gabrielle up she gets an idea into her head that maybe there is something Harry can do to help Gabrielle. He said it himself, he and Fleur can see Gabrielle for who she is and not what she has become. What's a little bit of minor experimentation, just enough to trigger Gabrielle, no big deal.

It takes some convincing on Harry and Gabrielle's part, that Fleur wouldn't hold it against him, that Harry is doing this because he'd be willing to for Gabrielle and not because he's being forced to by Fleur. Eventually though, with a lot of nerves on both their parts, Fleur coaches Gabrielle through oral and soon Gabrielle finds within herself a hunger she didn't know she had.

The following few days Gabrielle is convinced that her bust is growing and she feels that hunger more and more so. However repeated sessions aren't having the same results, besides diminishing returns there is also diminished satisfaction of Gabrielle's hunger - but increased attraction to Harry. Now it is time for Harry to be the one demonstrating oral, on Gabrielle. Another far more measurable growth spurt in height and bust but again it sputters out short of her complete maturation.

Gabrielle is struggling to control herself now, she's randy all the time and worse, her desire isn't just for Harry any more, she's dreaming of and fantasising about her own sister. She tries to avoid them but together they confront her, their tenderness and affection for her is heartbreaking, she blurts out that it's wrong, that she wants to be with them. Fleur begins by saying that she doesn't mind Harry and Gabrielle's actions but Harry interrupts to suggest that Gabrielle means something else by 'with them'.

Harry admits to Fleur that he isn't able to separate his feelings for Gabrielle anymore, he feels basically the same affection (not just the same lust) for Gabrielle that he does for Fleur. She is stunned, she realises that Harry seeing Fleur for who she really is (not for what she was, 'that hot Veela') was the same thing that made Harry the perfect man for her... and now has made Harry the perfect man for Gabrielle, because he sees who she really is too and cares about her. Certainly she is at least somewhat morally conflicted regarding the idea of incest, but in self-examined honesty she'd be lying if she didn't say she enjoyed the idea (or hadn't already had fantasies) of her and her sister sharing Harry in a threesome - now it would just be a little more sharing... so much for 'a little experimenting'.

The end culmination is an all-out threesome. Incestual femslash between the sisters with all manners of naughty eroticism that two girls and a guy can get up to with each other. The next morning they wake to Gabrielle fully matured into a beautiful woman, her dark despair banished and a heart filled with happiness, one they share in.

Now there's just the minor detail of the three of them coming clean to the sisters' parents...