Be sure you have a copy of Father-in-Law's birth certificate. Then have a nice talk with him where he tells you old family stories, places he lived. People he knew. Rumors about cousins. Wars the family has been in. If you can do so, record these stories. If not, write them down. Take pictures of him. Do this before it is too late.
You may have all ready done this. There are probably hints of where to look next in these stories.