At the time, 68 was a advanced age, and for the wife to take a abode in a convent after the death of her husband was the modern equivelent of "Assisted Living for Active Seniors". He was actually looking out for her in the will and the fact his daughter was a novice in the convent was a added plus.
Perhaps THAT is the reason behind the enigmatic smile that nobody considers...she had a good, considerate and loving husband?
The smile is that of a pregnant woman (as per another thread). Makes sense to me. That is pretty much the way an expectant mother looks. Sort of a "secret" smile of content.