There is going to be a fourth Ling and Ting book! As announced in PW:
Alvina Ling at Little, Brown has bought the fourth book in Grace Lin's Ling & Ting early reader series starring identical Asian-American twins. Together in All Weather will be published in spring 2016; Rebecca Sherman at Writers House brokered the deal for North American rights.Yay! This is pretty great! The third book, Ling and Ting: Twice as Silly (which I think is the best one so far) comes out in November so having this fourth book under contract is great encouragement. Now can it be called a series?