If she's always nice to her brother, Sarah could be one of the tender ones; those sensitive, serious children who think a lot and feel everything deeply. It seems quite reasonable that your daughter could be feeling a little less certain about her place in the family, and by by extension, her place in the world. She might be a little overwhelmed by the new responsibility and independence that is expected of her as a big sister.
Even if there were no new sibling, she could still be feeling more self-conscious because of changes in her body or something somebody said to her. A sensitive child can make quite a big emotional deal out of some little comment by a teacher or classmate.
There's nothing wrong with being shy. It's just the way some of us are put together. I was always shy, and still am, though I have a rich inner life and enough social activity to keep me happily engaged. More would probably exhaust me, both emotionally and physically.
If you let your daughter know you love and support her no matter what, she'll find ways to cope with her shyness. Forcing her to be more outgoing won't work if she's not outgoing. She might put on an act to please you, but there's something a little tragic in not being able to live one's own most authentic life.
There's a great book on personality types called "Please Understand Me." I highly recommend it to anyone who finds a child, spouse or coworker's way of being in the world perplexing. See more detail here: http://www.amazon.com/Please-Understand-Me-Character-Temp...