The 'Bro, Are You Flirting With My Sister?' meme originates from the popular Disney Channel animated series Phineas and Ferb, specifically the episode 'Backyard Aquarium.' In the scene, the character Baljeet is dressed in a traditional feminine outfit for a backyard show, leading Buford to aggressively confront Phineas by asking if he is flirting with his 'sister.' The meme gained traction online as a versatile reaction image used to depict irrational overprotectiveness, awkward social misunderstandings, or situations involving gender-bending tropes. It perfectly captures a blend of cartoon nostalgia and aggressive irony, making it a favorite for ironic shitposting and relatable sibling-themed humor across social media platforms like X and Reddit.