What should the first wife do, who has not borne children, and suffers from her husband's injustice and ill-treatment of her, and his focus on his second wife and her children?

We advise the sister to be patient and trust in Allah, for this world is but a transient passage, and what she has missed Allah will compensate her for. It is not permissible for the husband to discriminate between his wives; he must be just between them. Indeed, a severe warning has been reported for whoever inclines to one of them over the other: "Whoever has two wives and inclines to one of them over the other will come on the Day of Resurrection with one of his sides fallen." Likewise, the husband should not flirt with one of his wives in front of the other. We advise the sister to supplicate for her husband's guidance and to explain to him the impermissibility of his action.