I have a sister, 3 step-sisters, and a step-brother. I didn't grow up with any of the step-siblings, so I see them as extended family. When I go to their birthdays, I do so out of a sense of duty more than a sense of wanting to go to their birthdays.
I have immense respect for my sister though, and the feeling is mutual. We can get into fights sometimes if we're around each other too long, but in the end we love each other and support each other.