A smart question. The grammar book may say that in this circumstance the 1st character be pronounced the second tone while the 2nd and 3rd character remain the third tone. But in real life, especially in colloquial expression, the 3rd character is pronounced a NEUTRAL TONE, which means it doesn't have a tone. So a native speaker will pronounce like this: ní(2) jiě(3) jie(0). This phenomenon shows up widely in words with two repeating characters. Here are some other examples: 谢（xiè）谢（xie），我（wǒ）爸（bà）爸（ba），叔（shū）叔（shu）.

“姐姐”should be pronunced in "姐3姐0”，so,"你姐姐”should be "你2姐3姐0”

HI Dude im chinese! about this question....u forget one tone: 0 no tone(in a soft voce). so answer is 你3 姐 3 姐0. Btw my english lever maybe B2 so we can be parnter? tell me ur skype or whatsapp?

叠词一般最后发轻声，很多中文词都有轻声需要特别辨别，比如房子，爸爸这种。

that's a very good question because a lots of Chinese can't say why .actually ,when two tird tone together ,the first character will become second tone .it will be teached when we in primary school.haha for example :往返 打水 美女 雨雪 but in this question ,“姐姐”is totally a neutral words.just like someone said before 这是个好问题，因为很多中国人都说不上为什么。实际上，当两个第三调的音节出现时，第一个字就会发第二调。我们在念小学的时候就讲过。比如：往返 打水 美女 雨雪。 但在这个问题中，“姐姐”是一个完全的轻音，就像前面有人说的一样。