What is the term for two siblings born on the same calendar day, one or more years apart? I know there's a term, and I'd know it if I saw it, but I can't for the life of me remember it.
Aside from saying that you "share a birthday", I am not sure that such a term exists. If it does it isn't likely to be very well known. Restricting it to only match siblings is likely to make things less likely to match up perfectly.
Generally, special birthdays are given a modifier. Most of these are just off the cuff:
Other words that could work apart from "shared":
You could also go the other way with it and accentuate the negative:
protected by RegDwigнt♦ Apr 11 '12 at 14:25
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