In Shanghai Girls, Pearl is a dragon and May is a sheep. Do you think the two sisters, in their actions in the novel, are true to their birth signs?

I have to answer this question with a "yes" and a "no."  The reference to the dragon and the sheep is from chapter 1.  The reader learns that Pearl's birth sign is the dragon and May's birth sign is the sheep.  
If the question is simply asking if the two girls ever embody their birth symbols, then the answer has to be yes.  A general thought about sheep is that they are dumb and complacent.  While I don't think that May is stupid, she hasn't learned as many language dialects as Pearl.  Pearl has learned seven, and May has only learned two.  That seems to indicate that Pearl is the smart, motivated sister, and May is the less intelligent, more complacent sister.  
Another example of where I see each sister acting the part of their birth signs is at the end of the book.  Yen-yen asks May and Pearl to join her in a picket.  Pearl agrees to do it because she's a fighter, which is how I envision a dragon.  On the flip side, May gives a retort saying that as women they can do nothing.  They are powerless like sheep.  
I don't believe that the girls always act according to their birth sign, though.  There are times in the book when I see May as the stronger and fiercer fighter between the two sisters.  For example, it is May that refuses to have sex with the husband that has been chosen for her.  Pearl doesn't fight in this case.  Readers are told that she stares at a curtain while Sam is on top of her. In this case, she is meek and mild like a sheep.