Praise be to Allaah.
She has to bear it with patience as much as possible, and not repay evil with evil. She should respond to her co-wife’s provocations by keeping silent and keeping calm. If she can write her a letter expressing her views, this is good. Then after that she will not be responsible for what she (her co-wife) does, because she will have done what she is obliged to do.
Shaykh Muhammad al-Duwaysh
Adhere to the words of Allaah (interpretation of the meaning):
“and when the foolish address them (with bad words) they reply back with mild words of gentleness” [al-Furqaan 25:63]
“Repel (the evil) with one which is better (i.e. Allaah orders the faithful believers to be patient at the time of anger, and to excuse those who treat them badly) then verily he, between whom and you there was enmity, (will become) as though he was a close friend” [Fussilat 41:34]
Know that there is no better way of dealing with her than fearing Allaah concerning her when disobeys Allaah with regard to you..
And Allaah is the Guide to the straight path.