The assumed speaker is an older sister ( uchi no = my family’s). In Japanese media, the older sister/younger brother dynamic is often platonic, but internet remix culture sexualizes it ironically. The phrase’s virality stems from this tension.
The anime revolves around the daily life of Kousuke Oono, a high school student who lives with his little brother, Kazuki. The story takes an unexpected turn when Kazuki suddenly grows to an unusually large size overnight, becoming a "giant" at around 10 meters tall.
What about you guys, ever have one of those days where everything just seems to go wrong?
Why is this phrase so persistently funny? It breaks three unwritten rules of comedy and horror.