I had to look this up myself last week because I kept seeing the name and got confused. There is no character named Matilda Weasley in the original seven 'Harry Potter' books or the main play. Ron's parents are Arthur and Molly, his older brothers are Bill, Charlie, Percy, Fred, and George, and then there's Ginny. No sister named Matilda.
I think the confusion might come from the 'Hogwarts Legacy' video game. That's set in the 1890s, and there's a Professor Matilda Weasley who's the Deputy Headmistress. She's a Weasley ancestor, presumably. So if someone's talking about her, they're definitely referencing the game, not the books.
It's easy to mix up, especially with all the expanded universe stuff now. I've seen people on Tumblr making fan art of her and treating her like a book character, which just adds to the mix-up.