na

if they actually grew up together as brother and sister then I would say it's weird just like I would think it's weird for adopted siblings to have a romantic relationship. tbh I would find it less weird if two biologically related siblings were seperated from each other very young and ended up having a relationship later in life without knowing they were related. if OP's and step brother's parents got together when they were in their teens or they weren't around each other enough to consider the other as a sibling then I wouldn't find it very odd. Kind of awkward, but not too weird