Elizabeth Casse's second husband was the elder brother of her son-in-law (that is, the husband of her daughter by her first husband), and Elizabeth Casse had children by both her husbands (see the diagram below). So Elisabeth Dumontet's brother-in-law was her step-father, and she was an aunt by marriage to her half-siblings. And the children of Pierre Monet were related to the children of François Monet not only as first cousins, but also as half-uncles and half-aunts. (Genetically, the two sets of children shared 1/4 of their genes, the same degree of relationship as between a grandparent and grandchild, as between half-siblings, or as between a full uncle/aunt and niece/nephew.)
In other words, Elisabeth Dumontet was her own step-aunt! (That's not quite as impressive an accomplishment as being your own grandpa, but still plenty confusing.)
1699 François Monet ==== Marie Dumas 1661-1746 | 1675-1753 | +------------------+--------------------------------+ | | | | | 1712 1730 | | Jean Dumontet ==== Elizabeth Casse ==== Pierre Monet | 1659-1729 | 1696-1766 | 1704-1774 | | | | | | | 1732 | children François Monet ==== Elisabeth Dumontet 1706-1762 | 1717-1767 | | many children
All these people lived in Quebec, by the way (except the elder François Monet, who was born in France, and Elizabeth Casse -- or Corse -- who was born in Deerfield Massachusetts, and taken captive in a French and Indian raid at the age of 8, and had an illegitimate daughter before her first marriage, at age 16; but that's another story). I'm descended from a son of François Monet and Elisabeth (or Marie-Elisabeth) Dumontet.