...Like two peas in a pod/They do everything together...

A friendship 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

Literature

• Fred and George Weasley in Harry Potter

Real Life

• CS Lewis and his brother Warren were both Inklings.

Western Animation

• The Trope Namer is Twilight Sparkle's song about her fillyhood friendship with her brother.