On the face of it there seems to be no reason why you should not keep and enjoy the ring, as nothing you have told here suggests that the giver is not in her right mind and knew what she was doing.
I do, however, have one concern: you say you work for this lady and her husband. Is this an informal agreement, or are you employed by an agency?
I am asking this because I would expect an agency to have rules about whether their employees may or may not accept gifts from those the work for.
Obviously, no-one could reasonably object to your receiving a box of chocolates or the like, but a ring, if it is not costume jewelry but valuable is a slightly different matter.
My concern is that you may be laying yourself open to accusations of theft or of having used undue influence, if not by the lady herself or her husband then by their heirs.
You don't mention whether this lady has children, it sounds as if they either do not, or are estranged from them. If the latter is the case, you do need to be careful, as estrangements rarely last when it comes to inheritance!
If she has children whom you know, it might be best casually to mention to them that their mother has insisted you have this ring and that you are mentioning it to them because her insistance made you a little uncomfortable.
After my father's death my sister and I discovered that some silver spoons and our paternal grandmother's engagement ring were missing from the house.
We had no way of knowing whether our parents had sold or given these things away, or whether someone had helped themselves to them. We decided just to let it be and do nothing as we had no way of knowing what had actually happened to these items, but it was not a comfortable feeling, even although neither of us "needed" the things, but the ring had been promised to my sister and I was hurt for her sake that it could not be found.
