| Is this a taboo subject only to your Father-in-law or does it include the entire family? If it only pertained to your Father-in-law, you might get some information from another family member, like an Aunt or Uncle. If you do not know the name of the woman who bore the children, do you know the dates of the births? If you knew the dates, or thereabouts, you could search the Department of Vital Statistics.
If you do not have any information to start with, you could find a site or two or three (there are many) for people looking for their birth parents, for birth parents looking for their children, and for siblings looking for birth siblings. You would post your information and ask for any assistance possible.
You may never find out - if the secret has been buried this long. Then again, you might just come across someone who knows someone. It works often that way. The very best of luck to you. |