Mother Knows Best
Molly Weasley knows her children like the back of her hand. So when they try to argue or deny, she knows better. Molly Weasley also knows about the wonders of having siblings as she had twin brothers herself. So when Percy and her twins are arguing she remembers how her brothers would annoy her too. Molly Weasley also knows how you can tell between brothers to twins to lovers. So when she does her late night rounds in the Burrow she isn’t surprised to see Fred curled up against his twin.