[ossiblue]

I guess my question regarding this is, why do you think your FIL shared this secret with you? There are myriad other questions which follow, but without knowing anything of the substance of the secret, those other questions can only be formulated by you.

In your OP, you used many qualifiers about other family members, indicating that you really don't know if you are the only holder of this secret. Fair enough. That just underlines the question I posed--if you are the only one, why disclose it now, and why to you? If you can come up with a satisfying answer to that, it may guide you as to further inquiries with family members.

BTW, I'm another here who is anxious to hear what this secret may be.