Chikasugiru Futari

From the moment they were born, they've been together through everything—waking hours, sleeping, and even bath time. At this point, calling them "childhood friends" feels a bit too soft. No, these two are "Sh*ttyhood Friends." More than childhood friends, but less than lovers. Having come full circle, they're basically an old married couple, with a messed up sense of personal space.

(Source: Kadokawa, translated)