I think it definitely depends on the relationship between the stepparent and child. Growing up I had a couple of stepmoms. Both were and still are great women, but I never expected them to love me like their own children. I had/have a wonderful mom and although I loved my stepmothers, it was never close to the kind of love I have for my own mother.
It was totally different with my stepfather though. My biological father wasn't in my life very much at all growing up (tells you how awesome my stepmoms were that they made a point to be in my life, hah) and my stepfather took on the role of father when I was three. He adopted me and has been my dad ever since. I also have a half sister and I would be pretty butthurt if he were more loving to her than toward me.