(Spider-Woman III#18 (fb) - BTS) - Rosa and her identical twin sister sister Marie were sexually abused by their father at a young age (see comments).
(Spider-Woman III#18 (fb) - BTS) - When their father murdered their mother, Rosa and Marie killed him in revenge.
(Spider-Woman III#18 (fb) - BTS) - Slaying their father triggered the ancient curse of flesh without bone, bone without flesh. Marie and Rosa became a living skeleton and a suit sentient skin, forced to go through life together as one person to avoid detection.
(Spider-Woman III#17 (fb) - BTS) - With help from their grandmother, herself a witch, Rosa and Marie devised a way to rid themselves of the curse which was specifically designed to affect twins. They learned of the twin fashion models Donna and Bess Bliss who needed a housekeeper to take care of their morbidly obese sister Bonnie. Applying for the job as Rosa, the Alvarez sisters started to work for the Bliss family. They quickly sussed out the situation, learning that Bess and Donna were eager for Bonnie to die. They were also willing to sacrifice everything to stay young and thin.
(Spider-Woman III#17 (fb) ) - After
quitting the job as housekeeper, Rosa waited around the Bliss' condo
because she knew her resignation would cause a violent row. Bonnie got
so worked up, she suffered a fatal heart attack. When the Bliss sisters
allowed their sibling to die, Rosa approached them and took the twins to
see to their grandmother who lured them in with promises of never
gaining weight if they drank a potion she had concocted. In return, she
claimed she wanted their souls. (Spider-Woman III#17 (fb) - BTS) - When Bess and Donna drank the potion, the curse was transferred to them, causing 'Rosa Alvarez' to separate into Rosa and Marie again.
(Spider-Woman III#17 (fb) - BTS) - Two years later, Rosa, Marie and their grandmother picked up the mystic meddlings of Miss Itch and Mr. Scratch who had partially transferred the curse to the innocent Mattie Franklin. Because the heroine wasn't a twin, she'd inevitably die from the powerful magics. The Esteves family decided to prevent this from happening.
(Spider-Woman III#17) - Rosa and Marie Esteves found Mattie thanks to their sensitivity to the curse's magical signature. Mattie recognized Rosa because she'd shared memories with Donna Bliss, but had no idea she was a twin as well. The sisters filled Mattie in: if they did not find Flesh and Bones before sunrise, the curse would reduce her to nothingness.
(Spider-Woman III#18) -
Rosa took Mattie to meet their grandmother who confirmed her initial
diagnosis. They had no time to waste in locating Flesh and Bones.
Driving around New York City in a red Volkswagen Beetle, the Esteves
sisters and Mattie used their sensitivity to the curse to home in on the
hotel where Miss Itch and Mr. Scratch had taken their daughters. Marie
and Rosa waited outside for Spider-Woman to tackle initial resistance
and then moved in to use their magical knowledge to transfer Mattie's
portion of the curse back to the Bliss sisters. (Spider-Woman III#18) - When their grandmother joined Rosa and Marie, they told Spider-Woman to leave so they could deal with Flesh, Bones and Itch & Scratch. Though Mattie balked at the notion, she obeyed when the old woman told her that 'know more would be more than her mind could bear'.