I second Calmwaters... Your sister may be turning this into a "she loves me more" competition because of your significantly different lives, trying to make you and her more "even". In any case, do what YOU feel is right. If it hurts too much, stop contact. If you decide you want to get to know your birth mother really well, that's okay too, as well as anything in between. A family counselor can help you decide if you feel you need help.
Good luck. I was born 11-19-1991 as well. Maybe that makes us both lucky? It sounds like both of us have been blessed in our lives, though in different ways. :)