Catholic twins

Siblings who are less than a year apart in age. A satirical phrase playing on the stereotype that Catholics have many children very close together in age. My younger sister and I ended up in the same year in school, so everyone always called us the Catholic twins. A: "Oh, I didn't realize their kids were so close in age." B: "Yep, Catholic twins born only 11 months apart." Some people thought my brother and I were actual twins because we were so close in age and looked so much alike, but, nope, Catholic twins.