All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger.
If your wife has really committed Zina (adultery), then she has indeed committed a grave major sin. This sin is even graver if she did so with her uncle who is one of her Mahaarim (plural of Mahram). Therefore, she is obliged to sincerely repent to Allaah. For more benefit on the conditions of repentance, please refer to Fataawa 86527 and 87903.
In fact, you should not keep her if she had not repented because how would a Muslim keep a wife who violates his honour and dignity and who may give birth to children from other men? Since, you have already separated from her, then praise be to Allaah.
As regards the questioning of your daughters as to what happened, then we advise you to be wise when answering them. For instance, you should inform them that Allaah predestined that the marital life between you and your wife stops and that divorce is an ordinary matter that may happen between the spouses for one reason or another. If it is necessary to tell them the real reason, then you may do so.
On the other hand, you should know that in principle the fostering depends on the interest of the fostered child. So, if you fear that your daughter will be spoiled or harmed by being with the mother of your wife, then you should take the matter to this Islamic Committee so that it will study the matter and judge according to the benefit and interest of your daughter either in her being fostered by her grandmother or canceling her (grandmother’s) fostering and giving it to the one who is fitter to be her fosterer after her grandmother.
For more benefit, please refer to Fataawa 84618 and 90461.
Allaah Knows best.