The tricky thing is when the bio parents are still in the picture. I've been in my step childrens' lives since they were 2 and 4 (they're now 17 and 19). My own two children feature very differently in my life. My children only have 2 parents to give them love, whereas my step children have a mother, a father, a step mother and a step father. From that, it would almost seem unreasonable for me to treat my step children the same as my own as they do still have a mother of their own.
It's a very different situation when one of the bioparents is no longer around.
FTR, I do still involve my step children in our lives but now they're teenagers, it's become less, but the relationship is different.