I'm not going to let my boyfriend drive my sister's boyfriend home
Why wouldn't you let him do that?
I'm trying to say that he's kind of dangerous
You may think that, and that's fine.  However, don't you think you might make him act dangerously by being so controlling?  You should give the guy a bit of space to do what he wants.
I know what's best for him.
You are being too controlling. Your boyfriend is a grown man that has the freedom to make his own choices.