You should open up the conversation with him when he gets back:
Brother: "How was everything?"
Sister: "Great. Though he/she did chew up baby's new present. Oh well. Live and learn."
Then he may offer to pay.
But, and this may be the most important part, he then will know that his dog chews things up when he's gone (as people have pointed out, dog may not do that when brother is home). Then when he needs a sitter again, he can warn potential sitters.
And .. having this conversation with brother now makes the conversation in the future a lot easier when he asks you to sit again:
You: "just to be clear, I know dog chews when your gone. Anything else I should know? Food protective? Impatient with kids?"
Kudos to you for taking care of brother's dog.