Can You Marry Your Uncle's Wife in Islam?
Question

Is it permissible for a man to marry his uncle’s wife after their divorce? What would his rights be towards the kids, since they are his cousins? Thank you. Allah’s blessing be on you.
Praise be to Allah.Islamic Ruling on Marrying an Uncle’s Ex-Wife
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 uncle’s wife is not a Mahram (someone 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 prohibited relationship with his uncle’s wife. The devil could make something that is bad appear attractive to them, so it is essential to exercise caution.
Caution against Creating Marital Discord
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 Shari`ah in this case dictate otherwise.
Permissibility of Marriage if There’s No Suspicion of Misconduct
If worst comes to 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 uncle’s 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 Allah, then you will have a great reward from Him.
For further insights, explore these answers:
Is the Father-In-law a Mahram?
Is the father of one’s ex-husband a Mahram?
Is the wife’s sister considered to be a Mahram?
Is My Husband’s Stepfather My Mahram?
Is Brother-in-Law Mahram?
And Allah knows best.