Good luck with that one... Short of a DNA test (which can only exclude the father, not confirm) or a stack of letters recognizing the child (and likely if that was the case, probably paid off already), you’re only going to blow money on lawyers.
I came across a situation that was similar in people coming out of the woodwork.
Child of age barred from making claim. DNA was not relevant even if could be established. It was fact specific and depended on State Law, etc.
So this alleged heir can expect a fight from the siblings for sure. It will turn on very specific facts and Minnesota law.