Was I out of line? Cussed out my my sister's boyfriend?

You were within your rights to admonish him for his carelessness when pulling into the driveway. Many teens have a heavy foot when first start driving and will deny it until the end of time. The profanity was wrong. Trying to get your point across will not help when you anger that person with profane name calling. Now that the initial incident is over I would now be concerned he was driving so recklessly with my sister in the car. I don't think any parent over reacts when trying to protect their child. An immature 19 year old will look for any excuse so as not to take the blame for foolish behavior. I would approach him and tell him, you are sorry for all the profanity but you still believe he was wrong and not being as careful as he should be when behind the wheel of a 2 ton bullet. Maybe he can relate when thinking in those terms