So common in fact that they had to pass a law in 1907 to make it ok for a man to marry his dead wife's sister.
(Women didn't get the similar right until 1921.)
Boring history lesson:
Before 1835, if close relatives (including a dead wife's sister) were found to be married then the case would have to go to the Ecclesiastical Court who would decide on the matter. They would decide if the marriage should be voided (ie was not legally binding). But this often took a long time.
So, in 1835, Parliament decided that every marriage of close relatives would automatically be "void". What this means is that the marriage is not legally binding and is treated as though it never happened:
"And be it further enacted, That all Marriages which shall hereafter be celebrated between Persons within the prohibited Degrees of Consanguinity or Affinity shall be absolutely null and void to all Intents and Purposes whatsoever."
There were attempts to get this law overturned from the 1870s onwards. An interesting person at this time was Dinah Craik (nee Mulock) who published a number of essays and novels.
Her 1872 novel "Hannah" was all about this issue. She told the stories of three very different young women.
Hannah is a 30 year old governess. Her younger sister has just died in childbirth and her brother-in-law, Bernard, whom Hannah has only met once, invites her to his house as a “dear sister” to help raise the baby. Yet, two unmarried people of the opposite sex living together was seen as scandalous in society, despite their apparent sibling status – a fact referenced by several characters.
By the end of the novel, Hannah and Bernard have fallen in love and they leave England forever to go to France, where such marriages were legal.
The novel also has two separate subplots. The first follows Hannah’s maid who was tricked into marriage by her brother-in-law, James Dixon, after her sister died.
Grace, the maid, is a virtuous but badly educated woman unaware of the intricacies of the law. When her brother-in-law persuades her to marry him to raise her sister’s children, Grace agrees for the children’s sake not knowing that it makes her own subsequent son illegitimate.
Her argument, that such marriages are common amongst the working class who can’t afford to pay anyone to raise their children except a wife, blinds her to the fact that, though common, these marriages are technically illegal. When Dixon grows tired of her, he abandons Grace and her child, and Grace has no legal recourse to protest this. She is left as a disgraced woman.
It has been argued that this novel influenced much of the publicity that was subsequently used by activists to reform this law.
So, in 1907 the Deceased Wife's Sister's Marriage Act 1907 was passed which made it legal for a man to marry his dead wife's sister.
But what about the other way round? Well women only got the equivalent right in 1921.
This was due largely in part to the number of men killed in World War One.
However there were also concerns about the cost of war widows pensions and the increasing level of divorces (which were considered a v bad thing at the time).
From a speech at the time in the House of Lords:
"But circumstances have now changed, and, as in many other respects, they have changed in consequence of the war.
When the war took place many men, when they were called up, left their wives and families in the charge of their brothers. Some of these men were killed, and in many instances an affection developed between the brother in-law and the widow, and many marriages of this kind have taken place.
I believe that there are a number of cases in connection with the Ministry of Pensions which are awaiting decision as the result of this Bill."
But there were concerns about so many people getting divorced and it was considered that these marriages were more stable:
"At the present moment there is a perfect orgy of divorce going on; so much so that the noble and learned Lord on the Woolsack has had to devote his attention to it, and octogenarian Judges have been dragged from their retirement to cope with the ever-increasing flood of these cases.
I read in the paper today the statement that in one year alone, between 1918 and 1919, the numbers had increased by no less than 114 per cent., and they continue to increase at a still greater ratio.
It is a most surprising, and I think a most convincing, fact— or ought to be so— that, so far as I am able to understand, in not one single instance in which a man has married his deceased wife's sister has he been a party to these proceedings."
https://api.parliament.uk/historic-hansard/lords/1921/jun/28/deceased-brothers-widows-marriage-bill
When it came to war widows pensions, a war widow would get 20 shillings a week and extra for any children. But the pension stopped as soon as she remarried.
There was a financial incentive for the government to pass this bill and so we got the Deceased Brother's Widow's Marriage Act 1921.