It is intrinsically forbidden to speak negatively about another, this constitutes the basic definition of lashon hara (gossip). Consequently, it does not make a difference if the speaker and listener are good friends or relatives, husband and wife or parents and children – if they speak negatively about another it is lashon hara (evil speech). (See Chofetz Chaim 8:1)
Sadly, it is common for a husband or wife to tell one another about other relatives, workmates lashon hara. When parents are telling one another about their children there is generally a constructive purpose, but if there is none whatsoever then it is forbidden for them to relay to each other negative things about their children.
This is all derived from Miriam’s story, as the Chofetz Chaim writes in the Be’er Mayim Chaim. She spoke negatively about her brother Moshe, and her intention was merely to clarify the correct behavior for herself and Aharon. Furthermore, Moshe was the most humble of all men and certainly forgave her, but nevertheless she was struck with leprosy.
It is important to be aware of a leniency which the Chofetz Chaim mentions, although he leaves a question mark over its validity. It is common that a wife might want to tell her husband (or vice versa) something negative about another in order to get it off her chest, and by sharing her anxiety she hopes to reduce it. Or she might be hoping to get her anger out by sharing it with her husband. The Chofetz Chaim (10:14 in the hagahah) considers this to be a valid constructive purpose, since the speaker needs to calm down. Of course, the speaker must still ensure to fulfill all the regular conditions of speaking for a constructive purpose, which we will elaborate on in the future. Still, it is important for the speaker to verify that he/she really needs to tell the other this negative information just in order to calm down and not in order to denigrate the one spoken about.
Sefer Chassidim (Siman 64) also mentions this type of constructive purpose, albeit with a slightly different definition. If one speaks negatively about another and the listener knows that if he listens now the speaker will no longer want to tell others, because he is only telling it over to get it off his chest it is a mitzvah to listen to him, for otherwise he might try to reveal it in public.