Hey there,
I agree with what Dez and Sarah said, and I wanted to emphasize on remembering to stay calm no matter how your parents react. Your mom should know about connecting with people online, as Sarah mentioned, and if she's not naive, I would think something like this wouldn't seem completely out of the ordinary for her. The fact that your brother picked up people online a decade ago might not be in your favor when you do tell her, but maybe you could tell her that you know it's different, you're not being careless and that you're not just picking up this girl, you do actually love her.
I also agree that maybe writing a letter might be more effective than sitting down and talking, although eventually you will have to do that. Maybe to warm them up to see how they might react, casually ask them if they know anyone who's in an online relationship, a long distance relationship, or their opinion on either. Sort of testing the waters, so to speak.
Whatever happens, we're here for you, and as has been said, don't get too discouraged, because you are almost 18.
I hate to say it like that, but that is to your advantage.