I'm going to catch my younger sister with her boyfriend and then tell our parents.
Won't you feel bad about hurting your sister?
No, she needs to learn to communicate better with him. She doesn't communicate very well.
If you think your sister needs to communicate better, you should talk to her and explain that to her. Telling on her won't solve anything and will over worsen your relationship with her. Share your feelings openly to help her improve her social skills.
It is not just that. She is too young to be in a relationship and should be studying.
Is is normal to be concerned that your sibling is in a relationship too young. It is a delicate situation to want to help her but to also not offend her. Let her know what you think her priorities should be and if she doesn't listen, you can always get your parents to intervene to help.