What should we do?

My husbands bio dad passed away. This man’s life long partner didn’t know he had a kid at all (I.e. my husband). Since he passed due to health issues we want to say something to her and the only way to do so is by explaining who my husband is to his bio dad. We just want to know medical history for the sake of our kids an my husband.We just don’t know when to say something or exactly how to say it. Bc this will not only expose his bio dad for having a kid an not telling but also that he had cheated on her as well. This is over 41 years ago this happened.