What is the ruling on someone who speaks ill of people in their absence, claiming to be doing good and that Allah knows best the intentions, and what is backbiting and how can one get rid of it, and is a person sinful if their mother is the one who does that?

Gheebah (backbiting) is your mention of your brother with what he dislikes. It is one of the major sins that Allah has forbidden and warned against, like one who eats the flesh of his dead brother. Everything a servant utters is recorded against him, and the rights of people do not lapse; they will be taken from the backbiter's good deeds on the Day of Judgment. Advising parents is obligatory, and it must be done gently, kindly, and with good speech.