What is the ruling on one who performs the five daily prayers in the mosque and then curses people, and on one who swears falsely by Allah while praying, and on one who boycotts one of his siblings at home because he constantly talks about and insults people?

Slandering people and swearing falsely are reprehensible traits that are not befitting of someone who observes prayer, for prayer forbids indecency and evil. Slandering people is a grave matter; the bankrupt person on the Day of Judgment is one who comes with prayers, fasting, and zakat, yet he had slandered, falsely accused, and consumed the wealth of others. The wronged individuals will take from his good deeds, and if his good deeds are exhausted, he will take from their sins, which will be cast upon him, and then he will be cast into the Fire. Swearing falsely is a 'ghamus' oath (a plunging oath) that plunges its swearer into sin, and it is among the major sins for which there is no expiation according to the majority of scholars. Boycotting those who slander people is Islamically required, especially if it will deter them. It is obligatory for anyone who witnesses someone slandering a Muslim in their presence to defend them and to advise and forbid the slanderer. If the slanderer does not cease, then it becomes obligatory to leave the gathering.