Yeah, English people in Jane Austen's time felt there was nothing wrong with cousin marriage, but you know what WAS considered incest? Marrying your widow's sibling. Because that was your sister/brother-in-law, which meant that WAS your sister/brother! This was actually made illegal in the UK from 1835 to 1907/1921.
Before 1835, you COULD marry them, but if somebody challenged your marriage in court, the marriage could be voided. After 1835, you just literally couldn't legally marry in England.
The thing is, this did not stop people from marrying their siblings-in-law. It was very normal for a wife to die, and the wife's sister moves in to help take care of the kids, and then the sister and the widower fall in love, and marry. After 1835, couples like this would be forced to have essentially a destination wedding so you could be legally wed in someplace like France or one of the English colonies where it was still legal. If you weren't wealthy enough to travel out of the country you were in a spot of trouble.
Another fun fact is that a man marrying his sister-in-law was made legal in 1907 (with the "The Deceased Wife's Sister's Marriage Act"), but a woman marrying her brother-in-law wasn't made legal (with "The Deceased Brother's Widow's Marriage Act") until 1927???
This is why it's pointed out in Emma that Emma and Mr. Knightley aren't legally siblings!




























