Mother-in-law

When the Bob came home, his wife Diane was crying. "Your mother insulted me," she sobbed.

"My mother? How could she do that when she is on vacation on the other side of the world?" Bob asked.

"I know. But this morning a letter addressed to you arrived. I opened it because I was curious."

"And?"

"At the end of the letter she wrote:

'PS. Dear Diane, when you have finished reading this letter, don't forget to give it to my son'".