Option 1: Because of the inefficient postal service, his letters, ransom demands or even the news of his survival didn't make it to his wife. So after Months of uncertainty, she decided to move on.
Option 2: The wife knew and was boinking his brother anyway.
The premise is SUPER unrealistic. Why wouldn't they ever use voice chat?
Also, that old guy being so friendly with the young boy is weird. Like, it's supposed to be cute, but it gives me creepy vibes.