Shelley's first wife was alive when he met Mary, but the marriage had fizzled out long before. His wife drowned while she was alone, and it was considered a possible suicide. No one knows for certain if it was a suicide or accident. As someone who has walked alone in the woods and recently almost stepped backwards off a small cliff I would hesitate to make the call. That almost-accident was caused in part by a cat. I'm sure my cat wouldn't have stepped forward and admitted any responsibility for the accident.
But none-the-less, Percy was free to marry and said his vows with Mary.
Mary wrote Frankenstein as her part of a challenge that she, Percy, and his friend George created. George is mostly known simply as Byron today.
In 1817, Jane died. Jane had never married although she'd had a short engagement. Her engagement came about because she'd accepted a young man's marriage proposal at a party. By morning, the wedding was off and Jane was very upset. Sources say she didn't want to marry him because she didn't love him. She contacted her brother and he skipped church—he was the rector—so he could take her home.
Jane died in July of 1817. Pride and Prejudice had already been published. Persuasion and Northanger Abbey were published in 1818.
Mary and Jane lived different lifestyles. Mary's mother had died after childbirth, and Mary probably idealized the mother she never knew. Her mother had been a firm believer in following your own path regardless of societal norms and wrote about it.
Jane, on the other hand, grew up in a rectory, with seven siblings, and followed the path of society's culture.
Jane died at age 41. Mary died at 53.
Almost two hundred years after their books were published, both women's stories are known. Both have had biographies written about them. Movies were based on their books. Each woman so different. Each story so different.
The numbers on the calendar have changed, but people haven't that much. No two are alike and each has a different story inside them.