In chapter 2 we are told that she is 21 years old (I mistranslated it to 22). Besides being the protagonist's older sister It means that she is older than Him.
What do you want a 22-year-old to look like? With gray hair and wrinkles?
Last night I read that saying Capping was like saying "Stop hyping It" or "Stop Lying" and I use it because it sounded like saying "Don't exaggerate it" but shorter.