One must have been born of a sister in lineage and one must be born of a son from the same father.

This is a very insightful question. One of the main plot lines is that the five Bennet daughters would not be able to inherit their father's estate (and the income that goes with it) when their father died because the estate is 'entailed' to the male descendents only. Even Mr Bennet's wife could not inherit it and pass it on to her daughters. This arrangement of a long ago ancestor's will was still in force and could not be broken - although British legislation in the 1800s abolished these types of restrictions. Because the daughters could not count on inheriting any money except a small amount from their mother's own property, it was necessary for them to 'marry well', that is marry someone with enough income for them to retain their pre-marriage standard of living (and social standing.) And therein lies the motive for finding husbands, and the decision about whether to marry for love or for money. In the book, a distant cousin of Mr Bennet, Mr Collins, is his heir due to the entail. Since the property could only be inherited by a direct male heir, it would seem logical that this cousin could only inherit through a connection with an older male relative and therefore this male relative would also have had the surname Bennet. The property could not pass through a female relative to the next male heir, otherwise, Mr Bennet could have passed down the property to any future grandsons through his daughters and Mr Collins would have been out of the picture. Now, why did they have different surnames? I do not have a direct answer to this question. But, if you remember Jane Austen's novel 'Emma', there is a character in this novel named Frank Churchill. His father's surname is Weston; the younger man was adopted by some of his mother's relatives when his own mother died when he was an infant and his father could not care for him. In the adoption, his name was changed. Perhaps the same sort of name change also happened with Mr Collins. Another situation that could lead to changing one's surname is an inheritance. It was not unknown in Jane Austen's time to change your name to that of the family from whom you inherited a considerable estate. In fact, one of Jane Austen's brothers did this very thing upon inheriting from a relative.

fitzwilliam