If there are aunts and uncles, presumably some of them are siblings of your father-in-law, as closely related as his children... A technicality: Not sure in th

If there are aunts and uncles, presumably some of them are siblings of your father-in-law, as closely related as his children... A technicality: Not sure in the USA, but over here that's not correct. Relation is measured in grades being those the basic parent-child "distance". So you're related in the first grade to your parents or children, but in the second grade to your siblings (1 grade distance from you to your parents, another one from your parents to them) so, unless the father in law has a living parent, no one is more closely related to him than his children. So for lack of a better way of understanding, I would just ask a lawyer.