How did they hangout so much and not know they were stepbrothers? Did I miss something?

Children don't really talk about their families to each other and since it seems like they lived separately there was no way for them to find out

Thanks! Find it kind of weird that neither set of parents told them they would be getting a step-sibling.

My mom remarried and I never knew my step bro/sis nor my new dad's family...

Same my father remarried and it was years before I met my step-family and learned about the marriage