Posted By:
Max WederBill
One should always proceed with caution in accepting valuable gifts from un-related elderly people. There may be questions of capacity, and it is often amazing how relatives will suddenly be interested in an item, once it's true value becomes known to them, even after a gift is made. From a moral perspective, there is no question that you should fully advise her of the value of the items.
As an aside (and I realize you stated it was hypothetical), your fact scenario poses some interesting genealogical questions. You said you were in contact with the "great great aunt of Leo Angermeier". I believe that would be the sister of one of Leo's great-grandparents. If he was playing ball in 1910 or so, the family tree created by this relationship would have some unusual branches indeed for her to be the great great aunt and only be 84 years of age. 
Max