My opinion is that as long as you're subject to your parents, living under the same roof, 18 or not, you're bound to it. I recommend reflecting on your mom's reasoning.
As for an oath, you really would need to speak to a true priest and discuss the entire situation with him. He will guide you. Your parents, of course, can loose you from an oath you took to them, but, frankly, it really doesn't matter if you took an oath to them or not. As long as you are subject to them--and I agree with the above comment, as long as you live under their roof your are indeed subject to them--you have an absolute duty to obey them in all that is not sinful. And I can say with absolute certainty that it cannot ever, in any way, be sinful not
to chat with the people your parents have told you not to chat with.