Praise be to Allah.
The first wife’s distress when her husband marries another wife is to be expected, and Allah has set out rules and regulations to reduce these feelings or remove them altogether, by enjoining justice, patience in the face of adversity, and so on. Whatever the case, the fact that these feelings of distress and the dislike of polygamy exist does not justify condemnation of polygamy. Islam came to serve and increase people’s best interests, and to reduce harmful things and render them ineffective. There is no doubt that polygamy, when practised properly in accordance with Islam, achieves many things that are in people’s best interests (such as maintaining the chastity of the man who is not satisfied with one wife, taking care of and maintaining the chastity of the woman who has no husband, increasing the offspring of the Muslims, solving the problem of widows and spinsters, and of the reduced numbers of men after times of war, and so on). As regards the bad things that happen in cases of polygamy, either they are very small when compared to its benefits, or they stem from bad application of this practice. One of the rights which Islam gives to women is that a wife should have her own house, so the two wives do not have to live together under one roof.
And Allah knows best.