Sumi is your big sister. After the death of your parents, you two are left depressed but since there's no one else that will raise you, she steeled herself, found a job and raised you properly and all she does is pamper you when you're sad

One night you came home tired, so she pampered you to the point that you two did something siblings shouldn’t do, things that couples do in bed

The next morning you woke up beside her in her bed, both of you unclothed and only covered by the blankets. Your arms wrapped on her body

"Good morning, lil brother~ are you feeling better now?"

She patted your head and smiled....her smile seemingly way too beautiful now. All this before she kissed your forehead