Supergirl and Spider-Woman are both the female counterparts of iconic heroes who wear blue-and-red costumes, but that isn't the only similarities these two superheros share.
The Originals: Kara Zor-El and Jessica Drew were the first women to become Supergirl and Spider-Woman respectively. Both women became superheroes due to their parents trying to save them from radiation poisoning. Kara Zor-El was born on a chunk of Krypton that had been converted into the floating city of Argo after the planet's destruction. When meteorites pierced the atmospheric shield of the city, they unleashed Kryptonite radiation in the ground. Dying of exposure to the lethal radiation, Kara's parents sent her to Earth to live with her cousin Kal-El aka Superman in the city's only escape rocket. On Earth, she was found by her cousin and assumed the identity of Supergirl. Jessica Drew lived on a mountain that had high levels of uranium in the ground. When she showed signs of radiation poisoning, her father put her in a pod that infused her with spider-DNA since spiders had radiation resistance. Jessica hibernated in the pod for years when she emerged as a spider-powered adult. Despite having no connection to Spider-Man, she was called Spider-Woman. Both woman became iconic heroes for a while until it was decided they both had to die. Kara died fighting the Anti-Monitor, while Jessica died fighting Morgan Le Fay (Jessica was revived but lost all her powers in the process)
Psychic replacements: When Kara died and Jessica was de-powered, each was replaced by a new hero using the Supergirl and Spider-Woman names. Both women used psionic energy to mimic having Kryptonian and spider-based powers. Linda Danvers became Supergirl when she merged with a protoplasmic lifeform called Matrix. After the merge, Linda found that she had telekinesis; a power she used to fly, surround herself with a protective shield that mimicked invulnerability, boost her strength, and fire psionic force blasts. Julia Carpenter was tricked into being part of a government experiment to create superheroes. An injection of spider venom mixed with plant extracts turned Julia into the the new Spider-Woman with the power to create webs of solid psychokinetic energy. Her psionic powers also allowed her to cling to walls by psychokinetically bonding her fingers and toes to any surface.
Both Linda and Julia recieved new powers that ended up with the two getting sidelined. Linda briefly became an Earth-born Angel of Fire while she was Supergirl. She lost her angelic abilities, only to have them unexpectedly returned and then she was trapped in Hell. Julia became Madame Web when Cassandra Webb died in her arms and gave all her psychic powers to Julia. However a cosmic event involving all the spider-powered heroes in the multiverse caused the new Madame Web to fall into a coma and awaken after the event ended stripped of all her powers.
Third try was not the charm: Cir-El and Mattie Franklin were the third Supergirl/Spider-Woman iterations. Cir-El was the genetically engineered daughter of Superman created by an evil organization to be their own Supergirl, and Mattie gained super-strength and flight from a evil magic ritual and used those powers to assume the mantle of Spider-Woman. Both Cir-El and Mattie had unusual powers. Cir-El could fire a blast of red solar radiation from her entire body, while Matte gained the power to sprout psychically powered spider-legs from her back after fighting an evil version of Spider-Woman. However they weren't popular with the fans and both were quickly gotten rid of.
Back again: Kara and Jessica were brought back as Supergirl and Spider-Woman with new origins. Kara was now placed on a ship along with her baby cousin when Krypton was exploding. She was supposed to take care of her cousin, but her ship crashed into an asteroid and Kara was trapped in suspended animation for decades. She was eventually freed by the asteroid's destruction and she finally made her way to Earth where she became Supergirl. Jessica Drew was revealed to have been exposed to a bio-energy beam containing spider-DNA while in the womb due to experiments her father was performing for HYDRA. Her powers were returned and she once again became Spider-Woman.
