Surprising as it may sound, Matilda Weasley doesn't show up in the canonical 'Harry Potter' books, so there isn't an official pair of parents stamped into the story. The Weasleys' core family—Arthur and Molly—are the patriarch and matriarch, and their seven children are Bill, Charlie, Percy, Fred, George, Ron, and Ginny. So, if you meet a Matilda Weasley in fanworks, she’s usually placed into that extended web: granddaughter, niece, or cousin depending on who fans pair her with.
In fanfiction and headcanon spaces I hang out with, Matilda most often appears as a child of Ron and Hermione or as one of Bill and Fleur’s offspring; those placements make her a direct granddaughter of Arthur and Molly and a cousin to characters like James, Albus, Lily or Victoire, Dominique, and Louis. Other writers give her ties to the older siblings or to one of the twins’ later families, which changes her vibe—from mischievous twin-energy to a bookish, studious type. Personally, I love the idea of her growing up in that chaotic Burrow household: whether she's chasing spiders or reading by the hearth, she feels right at home.