The Weasleys, for all their seemingly poor and kind personas, are the heads of the largest and most powerful Magical Mafia in Britain, and one of the largest in Europe.
The various Weasley children are where they are and where they aim to go for various reasons (Bill works at Gringotts because having a good relationship with a bank would be beneficial, Charlie works in Romania because Dragons, Percy in the ministry because they're the government, and having people hugh up on the inside is ideal, etc). But to them, family comes first.
And Harry, whether he knows it or not, is family.
So they do what they can to help him, protect him. From his first year to his third, the Weasleys look out for the sweet boy, sending him food or guarding him from leeches, while trying to keep a low profile. Somehow, despite all their efforts, the kid keeps getting into increasingly deadly situations, and saving various Weasley family members, only cementing his position as family.
Then come fourth year, Harry goes and gets himself into the Triwizard tournament. But that's not the worst of it.
No, on top of all that, he goes and gets himself into a relationship with the heiress of their biggest rival in the European scene.
The Delacour Family.
Possible addon:
You could add a police angle as well, with Hermione being offered a internship or helping Susan and her clique try and help Susan's aunt and the DMLE identify the heirs to the mafia that they know are in the school for the Triwizard Tournament.
Hermione does it to try and improve her prospects in the future, or for a sense of moral obligation, trying to keep her friends out of it.