How can the second wife prove her marriage after her husband dies, as the marriage was not officially registered?
Question

If a man kept his marriage to his second wife secret, and had children with her, then he died without having disclosed this secret to his parents and siblings, or to his first wife and her children, then how can the second wife and her children seek their rightful share of the estate?
Praise be to Allah.In a number of questions we have stated that it is obligatory to officially register marriages, because that will protect people’s rights so that they will not be lost.
Please see the answer to question no. 129851 .
But a man may be forced to forego registering a marriage because he lives in a western country where plural marriage is not allowed and the one who does that will be penalised. In that case, we may say:
If the husband married a second wife in accordance with Islamic teachings, then it is easy for the second wife and her children to prove that, because when she got married there must have been her guardian and two men – at least – who acted as witnesses or he must have announced the marriage to some of his friends or relatives.
The husband may have left behind something in his handwriting confirming this second marriage, or he may have registered his children from his second wife in his name, and so on.
So the second wife should take some of these witnesses and whatever proof or other circumstantial evidence she has to prove the validity of her claim, and present it to those in charge of the Islamic centre in her city, and they can present it to the family of the deceased and prove to them that there are other parties who have rights in this regard, and they are entitled to their share of the estate.
The first wife of the deceased and her children must accept that, and beware of cutting off ties of kinship and refuse to give what rightfully belongs to their brothers, because Allah has ordained that all wives and all children have rights to the estate, so there should be no differentiation between one wife and another, or between one son and another. They are all children of the deceased and are all entitled to their share of the inheritance. The prohibition [on denying them their rights] is further emphasized if any of the heirs are minors (orphans), because devouring the wealth of orphans is a major sin for which the one who does that deserves punishment in hell.
Allah, may He be exalted, says (interpretation of the meaning):
{Indeed, those who devour the property of orphans unjustly are only consuming into their bellies fire. And they will be burned in a Blaze} [an-Nisa’ 4:10].
And Allah knows best.