Hilchos Lashon Hara chapter 8, section 2

Now we will begin to explain about whom the Torah forbids speaking lashon hara.

The prohibition of lashon hara applies whether it is about a man or a woman. There is no difference between one's own wife and another woman. Sadly, many people fail in this. They think it is allowed to say bad things about their wife and her family in front of their own brothers and parents' family. There is no permission at all to speak lashon hara about them, just like about anyone else.