...Like two peas in a pod/They do everything together
forms when two people discover they have common ground between them. It can be anything from political activism to a hobby.
Siblings usually grow up together, and so there's more chance to discover common ground.
Very common in Real Life
Not to be confused with Did We Just Have Tea with Cthulhu?
or Draco in Leather Pants
- Fred and George Weasley in Harry Potter