What is the ruling on criticizing the actions of relatives in front of one another, or confiding in family members about a husband's actions, or criticizing someone's appearance or behavior in a low voice, or criticizing one's mother to oneself; and is talking to oneself, perhaps with insults or curses, considered a sin?

Allah has forbidden backbiting and depicted its perpetrator in an ugly light. It is when you mention your brother with what he dislikes. If what you say is true about him, then you have backbited him. If it is not true, then you have slandered him. However, backbiting is permissible in certain situations for a legitimate interest, such as seeking redress or seeking assistance to change a wrong. Accordingly, mentioning the faults of a father, brother, or husband to a mother or sister is not permissible except for a legitimate interest, such as changing a wrong, removing an injustice, or seeking advice. As for criticizing a person in a low voice, there is no harm in it as long as it does not involve insults or slander about what is not true. As for criticizing the actions of one's mother or others to oneself without speaking, there is no sin in it, and Allah knows best.