>>>What if they have different moms (step-sisters)?
>>
>>Then they wouldn't be sisters :-}
>
>I'm not sure if step-sisters always call each other step-sister or simply sister?
>
>I did get the joke, just thought I'll try to defend the blonde.

Depends on the family. Most half-siblings refer to each other as sister or brother (share at least one of the parents). Steps it usually depends on whether they grow up in the same house or not. In most cases (IMHO) all are considered sisters and brothers unless for some reason they need to specific step or half...