Catholic Twins Meaning

(slang) siblings born within twelve months of each other.

Example: Used other than as an idiom: see Catholic,‎ twin.
2008, Shelly Laurenston, The Mane Attraction, page 139:
  â€œAunt Sissy!” a young voice screeched, then Sissy was besieged by five kids who looked barely a year apart. What his mom would call Catholic Twins.