This is totally prohibited to have boyfriends/Girlfriends in Islam! You must tell your father so she will truly stop and he will keep her in the house.

My sister has a boyfriend who she is intending to marry. However, she consistently lies to my parents about where she is when going out, she stay late out at night without my parent being aware. I have told her that it is all sin. I also heard that the boy has a past, relating to zina. Now, do I tell my parents what she is been up too and about the boys past if they want to get married.