I think there are several reasons for this sort of marriage other than the 'scandalous' version. =)

1) protection and safety of a widow and children - much needed at certain times and certain places.

2) a widower with children still needed to provide for his family and therefore needed someone to care for the children and home so he could earn a living.

3) keeping family land and whatnot in the family, and protecting the heir's inheritance.

4) better the devil you know... there may not have been many eligible men/women in the area.

5) Society - there are places where taking an available sibling as a spouse was encouraged, if not expected.

And, after spending time together at family events for sometimes many years, who's to say that there wasn't a legitimate attraction?

Just my two cents...

Lynne in MD