I didn't know ane are suppose to be the grow up sister and imouto are suppose to be the little sister o.o but this was pretty funny xDDD
Ane and nee are mostly older sister though you can use ane for just sister in general. If you want to make it clear that just sister is meant you put ane + imouto together and get shimai (the onyomi - Chinese pronunciation - for the two characters 姉妹). In Chinese it's the same characters, similar pronunciation, you write 姉妹 Zǐ + mèi for sisters, Zǐmèi. The only difference is, Chinese usually doubles up Zǐzǐ for older sister, mèimei for younger, and japanese leaves them singular with 姉 ane and 妹 imouto (the kunyomi or Japanese pronunciation of the kanji). I am studying Chinese, mostly Taiwanese dialect, coming from having learned a lot of Japanese, with a friend who's Taiwanese now studying Japanese. 姉 = ane, shi. 妹=imouto, mai.
Brothers are the same, ani (kyou) + otouto (dai) = kyoudai, and of course, it's similar in Chinese xiong + di = xiongdi