So I've been best friends with this girl for years. She has two brothers and a sister who is married and living in a different state. One brother is only a couple years older than she is, and this particular brother went to school with me before I'd even met her. One Summer, we were hanging out (she still lives with her parents) when this older brother came home. We hung out as a group for a little while, but at some point, things changed; he was inviting me out alone.
When I would spend the night with her, I would get a text from him asking to meet me somewhere, and we'd talk or just hang out, and each time, we got closer and closer (tickle fights, holding hands, etc.). One such outing, he had me sneak out of her house in the middle of the night, and we made a road trip to a different city. And he went up this really long road, and at the top was such a beautiful view of the river. He told me he had fallen for me, and he kissed me. One thing led to another, and we made love in his car.
As far as my best friend knows, nothing ever happened between us, and we're merely "acquaintances." I feel horribly guilty because prior to any of this, my best friend had confided in me that several of her friends met her brother and fell in love with him, rejecting her for him even though he wasn't the slightest bit interested. Am I a bad person for not having told her any of this? Should I continue seeing him (and yes, we are still seeing each other) or break it off? I don't think she would be happy, and I don't want to lose her friendship over this. I'd rather pick her over him if it came down to it, so should I break it off and find someone else or continue the way it's going?