Coming from personal experience, I did not like my step-mom or step-dad. I didn't like her because she took my dad from me one day when I went to visit him to go get married. Years later, they had a church wedding and she put a girl who is not related to us in any way shape or form in the wedding, and put me in charge of the sign in book. That was not a good day for me, but I respected my dad and did everything I could to try to get along with her. Besides, she gave me a little sister and brother who I love with all my heart, she can go jump off a bridge for all I care. My step-dad, he treated my mom like a slave rather than his woman. He even tried to keep me from going to see my dad when I wanted to see him. My mom and dad did not get along after they divorced so my mom went along what he would tell her. But again, I respected my mom and did the best I could to get along with him.
If you want to know how to get your step-child to like you (or who ever your referring to) make sure that the mom or dad is making them respect you and that the mom or dad that is not in the picture is not telling the child lies about you to make them not like you. At the same time, show that child that you care for them and don't show any kind of disrespect to the absent parent or the parent that is present. If you show disrespect, they will disrespect. Let the child know that you are not trying to replace a parent, but giving them an extra parent to love and rely on.