if you guys talk on a regular basis then I would at least inform him that you will be, or are, dating his sister. If you just show up he may be surprised. Or have her tell him. If you don't really talk much and he is more of just an acquaintance then fuck it. Pick her up, and when you drop her back off make sure she looks like she just took the ride of her life. When you send her home she needs to be walking bowlegged and hair all disheveled, makeup a mess, otherwise you didn't do it right. Good luck