Legal Question in Wills and Trusts in Illinois

If I understand your question correctly, the ex-in laws had only one child and the three adult grandkids are the children of the only child.

In that situation, those three adult grandkids would not be heirs to receive a share if there was no will (the son would inherit all. In such case, they would have standing to contest the will only if they stood to benefit under a prior will that did include them.

If the three grandkids are from a predeceased child, then yes, they would have standing to contest. Whether they would have valid grounds depends on facts and circumstances.

Assuming your ex's parents were in Illinois, from the facts as stated and understood, it does not appear the grandchildren have any basis to contest the will, but an attorney would walk through various questions with you to better develop the background and see if there is any course. If you do not have a copy of the will, in Illinois it should be on file with Circuit Court Clerk of the county in which your ex's parents resided. It would be best to have it for a consultation.

Perhaps to help give you some peace of mind, first, in my experience, generational lines are followed in most plans. In other words, most people leave the bulk of their estate to their children and usually make grandchildren alternate beneficiaries in the event anything happens to a child. I suppose the expectation is that if there is anything left the child will then have a plan that provides for her or his child(ren). Second, even state law, in the absence of a will, would leave things along those same generational lines. Third, regardless all this, your ex's parents could choose to do anything they want in developing their estate plan. So, while I expect the will probably left the estate to the child, followed by the grandchildren as alternates in the event the child had not survived, there is not anything that would compel that.

Considerations to review with an attorney include: whether there is evidence of a prior will; whether your ex's parents were competent to prepare the will in question; whether there is evidence in anyone unduly influencing them in the preparation of the will; whether "only son" means only child ever or only surviving child at this time; ...