All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger. We ask Allaah to exalt his mention as well as that of his family and all his companions.
Islam urged people to have good manners and morals, and treat the Muslims based on the principles of Islamic brotherhood. The Prophet said: "Fear Allaah wherever you are, and follow a good act after a bad one, it will wipe it out, and treat people with good manners." [At-Tirmithi] Furthermore, the Prophet said: "A Muslim is a brother of another Muslim, so he should not oppress him, he should not forsake him, nor should he hand him over to an oppressor. Whoever fulfils the needs of his brother, Allaah will fulfil his needs; whoever brings his (Muslim) brother out of a discomfort, Allaah will bring him out of the discomforts of the Day of Resurrection, and whoever conceals the sins of a Muslim, Allaah will conceal his sins on the Day of Resurrection." [Al-Bukhari and Muslim] A wife with respect to other wives [of same husband] is included in the above directives, and any wife who is enabled to act according to the above is really enabled to much good; it might be that such wife will get the greatest reward, as in general the wives of the same husband jealous of each other and try to harm one another. If a wife is patient over this and treats other wives kindly, then she will surely get a great reward. It is confirmed that the Prophet said [regarding wives of same husband]: "Your reward will be according to the hardship and difficulties you experience [from other wives]." [Al-Bukhari and Muslim] Generally, a wife envies other wives of the same husband because of the beauty or children that they have, or because of other reasons, so this wife should fear Allaah and not envy other wives, as envy has unpleasant consequences, and she should be content with what Allaah predestined for her. It is confirmed that the Prophet said: "If you are pleased with what Allaah predestined for you, you will be the richest, and if you treat your neighbour kindly, you will be a real believer, and if you love for the people what you love for yourself, you will be a real Muslim." [Ahmad and At-Tirmithi] So, a wife should put this narration as the foundation in dealing and treating other wives of the same husband, and she should be content with what Allaah predestined for her and should not envy other wives. Besides, she should treat them kindly, and she should love for them and their children what she loves for her own self and her own children. If she does so, Allaah will love her, the other wives from the same husband will love her, and all other people will love her.
Allaah Knows best.