When was the step-sibling acquired? Have they been family long or only for a few weeks? They're likely to gain step-sibs every time a parent remarries. Could get crowded.
But a new relative isn't the same as someone you've grown to think of as your brother or sister.
I may eventually finish writing a story based on reality -- they bought my big split-level house to move into -- of a widower with three kids marrying a widow with four, all 18+ because LIT rules. It'll be Group Sex, not Incest, because no blood relatives fuck each other, only their brand new step-kin -- sibs and parents alike. With threesomes.
Expanding on the OP, let's have widower Dan (with two young kids) marrying divorcee Ivy (with two young kids) who dies soon so Dan marries widow Sheri (with two young kids.) Fast-forward a decade till the kids are all 18+ and horny. Blood and non-blood, they suffer and enjoy the usual desires, angst, manias, and discoveries.