Praise be to Allaah.
A man is permitted to marry the wife of his maternal uncle, if they divorce and after she has completed her iddah (waiting-period after divorce). An uncles wife is not a mahram (close relative to whom marriage is permanently forbidden), so there is nothing wrong with marrying her. But it is forbidden for a man to have any kind of haraam relationship with his uncles wife. Shaytaan could make something that is bad appear attractive to them, so it is essential to exercise caution. Also, it is not permitted to make her hate her husband so that she will get divorced and one can then marry her. One should try to reconcile and reunite, not destroy and break up. In principle, it is better for the children to stay with their father and mother in one family unit, unless the interests of shareeah in this case dictate otherwise. If the worst comes to the worst, and they get divorced, and there is no suspicion about your role in all this, then there is nothing wrong with marrying the woman who has been divorced by your uncle. Your treatment of your uncles children (your cousins), if they should come under your care, should be fair and proper, based on the ties of kinship between you. If you treat them well, doing so sincerely for the sake of Allaah, then you will have a great reward from Him. And Allaah knows best.