You were sorted into the Hufflepuff house along with your really overbearing and over protective brother who was none other than Cedric Diggory, the pride of Hufflepuff and Mr. Perfect according to most people. People thought and expected you'd be like Cedric, calm, chill and easy going but you were the complete opposite of that

You were a raging and wild spirit who wouldn't be tamed and you thought you had a free will to do whatever the heck you wanted. Yes, you did like helping people in need and you weren't a bad person at heart, in fact one even might say that you have a heart made of gold but you just have a lot of anger issues and it clouds your good personality. But no matter how many times you get angry Cedric being the big brother he is will always pinch your cheeks by the end of the day and act like you're doing something adorable, because in his opinion, you getting angry is really cute and endearing to him. You look so precious, he can't resist pinching your cheeks and smiling at you

You didn't like being babied at all, it wasn't like you were a 5 year old who needed to be supervised 24 freaking 7. Cedric would always stick by your side no matter what and he'd make sure you only have girls as friends not guys either. He didn't want anyone trying to corrupt his innocent little sister. And if you liked girls, well... you won't be having friends for a while and you can thank HIM for that. He doesn't see it as manipulation, he sees it as looking out for you and making sure you're safe, he's doing a job as a protector and your brother

Now, even though Cedric might seem like a nice guy but if anyone is caught flirting with you or trying to date you, surprise surprise, they've somehow managed to land up in detention and being the Head boy of the Hufflepuff house most certainly did have it's perks. And being friendly with all the Hufflepuffs as well was a bonus too, those little snitches tell him everything, they're willing to do literally ANYTHING for him, especially Ernie Macmillan and he'd threaten them to leave you alone and hex them and make them sorry for daring to even think of you. Pretty soon, he's earned the rep of being a scary brother to you when someone tries to ask you out, however you've managed to capture someone's heart and he wasn't willing to just give you up like that. Namely the boy who lived, Harry James Potter himself

You were in Harry's year and when he first found out Cedric had a sister, he was surprised and even more surprised when you turned out to be nothing like him. If at all someone committed a foul in a Quidditch match, instead of being all kind and nice and friendly about it, you started hurling insults at them calling them 'dimwitted trolls' much to Ron's amusement followed by a string of jinxes and hexes. One reeling question he's had in his mind ever since he's met you is how in the name of Merlin did you get sorted into Hufflepuff?

Later on, he saw your kind and helpful side when you offered to teach him Divination since he was utterly miserable at the subject and both of you bonded during that time. Luckily, your brother wasn't there since he was off at another class and you had some time to spare. You and Harry started hanging out more and he's started growing feelings for you. He honestly doesn't give a damn about you being Cedric's sister, he likes you, he wants you, he gets you. That's the end of that

Surprisingly, I can't see Cedric fighting Harry when it comes to you because he knows Harry is a decent and a good person. Harry wants to directly start dating you without telling your brother but Hermione told him that Cedric would find out anyways, and it would turn out to be a mess after wards. So, Harry first decided to tell your brother that you and him were now dating and he really likes you. He was prepared to use hexes and jinxes on him in case things go south and say, what if he uses the Imperious charm on him and then obliviates his memory? What? He's only doing this for your love, he loves you too much and he can't bear to lose you. He's willing to do whatever it takes to make you stay with him, even if he has to give YOU Amortentia

Cedric actually smiles and asks him if they both could talk for a bit leaving you all alone to dwell on your thoughts. You wondered what the heck was going on inside the classroom they went in and you couldn't go in since they'd bewitched it. You wondered if he was going to kill Harry now or something and he wouldn't be the Boy who lived anymore but he came out, looking really pleased and happy about something and Cedric came out smiling as well and both of them seemed to be in good spirits? Did they use tickling charms on each other or something?

What you didn't know was that Cedric didn't threaten Harry one bit, he just asked him to look out for you but did subtly threaten him if he ever broke your heart. Harry then replied that he wouldn't dream of doing such a thing since he really loved you. So, there you have it folks, you used to have only an overbearing smothering and overprotective yandere for a brother, congratulations, you also have another one in the form of your significant other