my dd did exactly the same thing when ds was born - she was 21mths old at the time. i can tell you it was lovely (NOT) when driving and ds started crying then dd would start up - arhhh music to a tired mummy's ears lol.

anyway she did grow out of it quite quickly, i always reassured her that her brother wasn't crying because he was sore or hurt it was because he was "talking" to us. i also read to her lots of books about baby's and also talked to her and reassured her that everything was fine.

hopefully you can find something to calm charlie down and help him settle when toby is crying.

good luck