My two year old cousin just was in a wedding, she is super shy around large groups BUT her 4ish year old brother helped her all the way down the aisle.
If the girls can go together that will help, unless both are super shy. Have someone they know really well (uncle, aunt, grandpa, you) in the back to reassure the girls and send them down OR to wait with them if they end up not going down the aisle.
For sitting through the wedding have some quiet toys for them to play with like; stickers, etch-a-sketch, Magnadoodle, books, pipe cleaners, magnetic books (just saw them at Wal-mart in toy section) and maybe some snacks that won't create messy faces/clothes.
My daughter is three and will be a flower girl in Sept, I am in the wedding but hubby will be sitting up front (along with grandma and grandpa since it is my brother's wedding). I am going to put the above suggested toys in an open basket where she will be sitting with daddy, I am going to show her where the basket is before the wedding so she knows there is something waiting for her (hopefully that will keep her interest since some the toys will be new and she will go all the way down the aisle to check them out).
I agree with the other mom, let the bride know that if the girls are having a bad day or frightened by all the people to just skip it. Have to be flexible when kids are in the wedding.