To answer your question, no they aren’t siblings or step siblings.
It’s established from the very first chapter, if not, from the chapters so forth on, that they’re neighbors who grew up together because their families are close.
Although a pretty big age gap ngl. They quite literally only participate in intimate activities together when mc is older and of age.
Also read the tags. Most stories will usually include genre tags—if you’re really concerned or kind of want a refresher on the story’s settings.^_^

So wait are they blood Brothers or are they step siblings. Im not even sure myself in over here second guessing myself but I don't know anymore.