Dating my best friends sister (Bakugo x reader)

Backstory

*Before Kirishima and Y/N were born*
Two parents always wanted to have a child, more importantly a son. They didn't want a daughter thinking she would be weak and quirkless. Once the couple found out they were expecting joy ran through them. But once they found out what they were having the joy they once had faded. They were expecting twins, a boy and a girl. The couple decided to keep the girl so their son can have someone to play with. Once the twins were born they refused to hold their daughter, even for a second! The twins grandmother went to visit the new parents and the newborn twins a few hours after they were born(I don't know how long you see a baby after it's born so bare with me). The grandmother held the girl and suggested a name to the new parents. They listened even though they didn't want anything to do with their daughter. She suggested the name Y/N. The parents went along with it not caring what name she had as long as their son had a nice name.
*Time Skip to when they were 5*
Y/N and Eijirou(I don't want to use his last name since they both share the same last name) was with their grandmother in their shared bedroom getting ready to go to sleep. " Do you want to hear a song before you two fall asleep?" asked their grandmother. They immediately nodded cuddling close to their grandmother waiting to hear the song she had to song.

(Iduna is their grandmother while Y/N is Elsa and Kirishima is Anna)
From that day on Y/N remembered that song for years to come.
*Time Skip again to when Y/N gets kidnapped*
Today was Kirishima and Y/N's 7th birthday. They both got a necklace from their grandmother of two halves of a heart. Kirishima had one side that said Bro, while Y/N had the other side that said Sis. Their grandmother told them that when they put the necklaces together the song she sang to them two years ago would play. They tried it out and it worked, they heard the beautiful melody of the lullaby. The twins went outside when no one was watching them and while they were playing Kirishima turned around to go get a ball to play with his sister when he heard a scream. He turned around just to see his little sister(Y/N is younger than him by 10 minutes) being grabbed by an unknown man in a black hoodie. He yelled at the man activating his quirk to try to free his sister from his grip but the man was stronger and threw young Kirishima back where he was. He screamed for his grandmother and parents to save his sister but they were too late. Y/N was taken. From that day on he blamed himself for not saving you. So he made a promise to himself that whenever Y/N was found he would be manly for her and be the big brother he was supposed to be.
522 words
I thought of this for a while now and decided to write it.
I hope you enjoy this book!