I wrote previously on sin-eaters here:

What were 'sin-eaters', and when did the tradition die out?

and have been researching the subject ever since, with the result that I have considerably deepened and nuanced my views on the topic. Hopefully, the results of all that work will be available in November in the form of a popular version of what will eventually become an academic paper – but in the meantime I should probably start by saying that the general thrust of my research has suggested that in fact sin-eaters never actually existed, and that the references we have to them are either caused by misinterpretation – one key characteristic of the entire tradition is that every account of "sin-eating" we have comes from outsiders to the communities that supposedly practised this custom – or were outright inventions.

With that said, I can certainly confirm that, with one solitary exception, every supposed account concerns male sin-eaters. Typically they were portrayed as outsiders even to the communities they worked within, not least because, in "eating" the sins of others, and thus taking them upon themselves, they supposedly became increasingly and catastrophically burdened with sin themselves. The earliest description to be published (not, to be clear, the earliest to be actually written) dates to 1836 and describes the sin-eater of Cors Fochno (Borth Bog) on the mid-Wales coast, supposedly reported by a passing English visitor who passed through the area in the late 1700s. I excerpt from my current unpublished draft:

The cottage where the candle burned was not quite the welcoming inn that the visitor had dreamed of, and it held horrors of its own. As the Englishman drew closer, "hoping for lodging," he saw that a corpse lay just inside the door, with a plate on its chest scattered with bread and salt. Then came "the sounds of wailing within, and soon a woman came out into the dead night, late as it was, and cried a name at the top pitch of her wild voice." Receiving no reply, she gathered up an armful of straw from the floor, set it down by the side of the path and lit it, "the usual signal of a death in the house." And, after a lengthy pause, the traveller caught the distant shout of a response, carried on the blustery coastal wind, then "saw at last the motion of what seemed a foggy meteor moving towards their standing point."

Some moments later, the shape resolved itself into a "wretched being" – a tattered wraith of a man who ducked into the woman's cottage. This, the visitor discovered later, was the local sin-eater, an outcast who "lived alone in a hovel made of sea-wreck and nails of such, between sea-marsh and that dim bog, where few could approach by day, and none dared by night, whether for the footing, or the great fear, or at least awe, which all felt for the recluse."[Source: Joseph Downes, The Mountain Decameron, 1836]

As for the solitary account of a female sin-eater: this is an anomaly in several ways, not only for the sex of the protagonist but because it comes from the fenlands around Cambridge, not from Wales. The account was sourced by Enid Porter, curator of the Cambridge and County folk museum from 1947 until her retirement in 1976, and published in Folklore in 1958. Since this account is also the only one we have that purports to tell us how a sin-eater was made, it is well worth reading. It appears to date from around the 1820s:

According to Porter's version of events,

an old woman who had died in 1906 recorded how, as a young schoolmistress in Little Ouse [a hamlet on the river of the same name, between Ely and Downham Market], she learned how the sin-eater, who, incidentally, was shunned by all the villagers, qualified for her profession. She took a large dose of poppy tea to render her unconscious. Neighbours sent for the minister, who on seeing her, said it seemed she would not recover. He read the prayers for the dying and gave her absolution. Soon after his departure the woman sat up and gradually recovered. She was then assured by her friends that as she was now free from her own sins by virtue of the absolution, she was now free to take on those of others. The sin-eater, after eating the half-round of bread and the little pile of salt placed on the shrouds of the dead, would receive as payment thirty pennies, which had been dipped in whitewash to make them resemble silver.

I daresay I don't need to interpret the veiled reference to "30 pieces of silver" for you.

I hope this helps – and that my conclusions are not too disappointing.