Mom vs. Dad

My mother and father got a divorce when I was three years old. My whole life I have wanted to meet and know my dad and finally my twin brother found him by Internet when we were seventeen. My mom did not and does not approve of us seeing or talking to him, for whatever reason, I am not sure. He surprised me at my high school graduation by being there, which my mother does not know. Then last weekend at my grandpa's I finally got to really meet him and my grandparents along with the little sister I never knew I had. My mom does not know anything about me seeing or talking to him and would probably kick me out of the house if she knew. After meeting my dad and the rest of my family I have decided that I want them to be a part of my life, but how do I tell my mother? Do I tell her or just keep on going behind her back? I have always been truthful to my mom, but I don't understand why she doesn't want me to see him, therefore I try not to hurt her and not to hurt my dad. Now I am stuck in between two people and I don't think this should be on my shoulders. I love my mom, but I want to love my dad. Should I confess and make a stand to my mother? Or should I lie low? I wish someone could give me some good advise, because I am being torn apart.