The Islamic Shari`ah identifies certain types of women Muslim men cannot marry. Among these is the ex-wife of one’s son. Thus, a father cannot marry the ex-wife of his son. But this prohibition does not extend to uncles, as the Qur’anic verse that enumerates the types of women prohibited in marriage does not speak of uncles in this regard.
Answering this question, Zienab Mostafa, a prominent Muslim scholar and preacher, said,
Almighty Allah says, (Forbidden to you are your mothers; your daughters; your sisters; your paternal aunts; your maternal aunts; brothers’ daughters; sisters’ daughters; your mothers that have suckled you; your foster-sisters; mothers of your wives; your stepdaughters who are in your guardianship, (born) of your wives to whom you have gone in, but if you have not gone in to them, there is no blame on you (in marrying them); and the wives of your sons who are of your own loins; and that you should have two sisters together, except what has already passed; surely Allah is Forgiving, Merciful.)(An-Nisaa’ 4:23)
In light of this verse, if the ex-husband is not related to the man she(the woman) wants to marry as a father or son, even by way of fosterage (rada`ah), then the marriage is permissible. If this man is only an uncle of he ex-husband, then the marriage can take place. The woman also need to be advised that this could only be done after the proceedings and consequences of divorce come to an end.
On a different note, marriage in Islam is not a ritual; it is held in a civil ceremony where the agreement of both parties is confirmed and the marriage is declared to the community. A registrar appointed by the government registers the marriage to make it official. So, marriage in Islam is a civil marriage in a way.