I know toddlers aren't well known for expressing their feelings in an articulate way, but how do you get them to open up?

Our 2.5 year old daughter has been doing this thing lately where she tries to dodge a lot of topics by answering with the phrase "Calvin (her brother) says ga-ga." This is weird for a number of reasons 1) She has a huge vocabulary so it's odd that she picks such a baby-ish phrase and 2) She's deflecting attention away from herself and to her brother because she's uncomfortable. Usually she says it if we're praising her for something she knows is a big deal (potty training), discipling her in some way (making her apologize), or to avoid anything she doesn't want to talk about (Santa - ha!).

I'm sure this is something that will get better as she gets older, but in the meantime I feel bad that she's uncomfortable and not able to express herself! Any good books or other advice?