Hilchos Lashon Hara chapter 8, section 2
Whom does the Torah forbid speaking lashon hara of? It applies to man and woman, whether one’s own wife or another woman. Many people think it is permitted to speak of a wife or in-laws to brothers or relatives. Actually, there is no such distinction unless one’s intent is for a future benefit and not to disparage them. Also, no falsehood should be included in what one says. The rules explained later in chapter 10 (paragraph 13 onwards) all apply here.