I made up my own word for different aged siblings that share a birthday or have birthdays that are hours apart but cross over to the next day from different age groups. I call them bypass-twins. They're virtual twins that bypass eachother's gestation on different years. They both celebrate their birthdays together but they're different ages. That is the closest compound word I can come up with that doesn't require whole phrase. The trouble is I would need to explain the word because I made it up. I would also use the made up word bypass-twin to describe friends that share birthdays because they share my birthday but they're also from different gestations, even if they're the same age as me because they come from a different womb.
I call my Chloè and Nathan bypass-twins inside my head. I don't feel comfortable saying my made up words with my mouth.
Nathan's birthday has been very interesting, his first birthday was days after the Myanmar military coup, millions of people in Myanmar were protesting. Now there is a balloon dispute between China and USA.
