I'm attracted to my 10yo stepdaughter?

I'm 26 my wife is 34 we have been married fo 1 and a half yrs. She has 3 kids from previous relations ages 14(boy) and 2 girls ages 10 and 5. Over the past year I've been finding myself becoming extremely attracted to my 10yo. I have never had an attraction to any child in the past and no child but her presently. I didn't have these feelings in the beginning and i have been in her life since she was 7. I find myself now checking out her but and getting very aroused just from the site of her I guess its just me seeing her body Develop that is doing to me. I believe that she has kinda en cited these feelings to a certain extent. I have noticed that when her mom isn't home she comes in my room with her panties and a t-shirt on and she will sit on my lap and talk to me. One time she called me in her room and when I came in she was naked She only does stuff like this when her mom is not home. She always wants to be around me if we are in public She always wants me to be holding her hand and if I show any affection to my wife or her younger sister she gets very mad. I have never put my hands on her in any way that would be considered inappropriate and I would like to believe i never would as I love my wife so much and would never hurt her. But my feelings for my stepdaughter are so strong now I don't know what to do. What should I do?