17 year old sister.

So , My sister is staying the week and her dad specifically said he dont want her taking off from my house. She has never done that before anyway. But she told me that this weekend is her bf birthday. They been together 4 yrs and she goes to his house to see him taking a train there. Well she asked if she can go see him sunday when he is off work from my house so her dad wont bitch at her about it. I feel like either way she will do it and i offered to pick her up etc. I do have respect for her dad though and god forbid something happens to her i wouldn't live that down. She is almost 18 and is gonna do what she wants. Should i let her go and be her big sister or try to be parent and tell her parents.