Japanese Language Trivia of the Day:

Once again, this is a word that I've learned because it applies to me, and I've never thought it was anything special. But it's one of those words that will win you an impressed reaction if you have the chance to use it.

年子
としご

toshi go

Rikai-chan will tell you that 年子 means the second child born within a year, but by that definition, the second in a set of twins could be called such, right? According to Japanese dictionaries, it means siblings who are separated by about a year, and in actual usage, there's some room to stretch that year out. I was told that my sister is 年子 even though she was born fifteen months after me. The general rule of thumb that I use is if the pregnancy turnover rate is fast enough to elicit raised eyebrows, low whistles, or off-color jokes, you can use 年子.

Usage:
When somebody asks me how old my sister is, I say "年子です。"
If somebody asks me 何人兄弟, I say "兄*と年子の妹がいます。"

* Edited to reflect the wisdom of the comment below