Should i intervene????

My sister is HELLA protective over my niece. For context, she is like 15 or 16. They don't let her go anywhere by herself. She has had no outings or hang-outs with her friends in her life (idk if she ever sneaked out). She is not allowed to go to any friend's birthdays and no friend is invited over at her birthdays. In conclusion- 0 outings except for with family. Oh and did i mention that they drop her off and pick her up from sch too which is not very far?

For more context- I accidently read her diary (IK ITS VERY WRONG I AM SO SORRY MY BABY BUT AUNT COULDN'T STOP HERSELF) and it make my heart sink. She does not like it at all. I did some research after that and found out all the info in the above para. As an Aunt i am kinda worried for her.

How would this affect her once she gets her freedom? Is it really serious or just a teenage thing? Should i intervene??