1. Peter is not as tall as his brother.
   This sentence is already correct. It's comparing Peter's height to his brother's.

2. The old restaurant is less popular than the new one.
   The word "unpopular" is a double negative here, which is incorrect. Instead, use "popular" with "less" to make the comparison.

So the correct sentences are:

• Peter is not as tall as his brother.
• The old restaurant is less popular than the new one