After doing some digging around, I finally found out the identity of Rey’s father. It turns out his relationship to both Rey and Darth Sidious is a lot more complicated than we thought. It turns out Rey had no grandmother and Palpatine had no lover. After Palpatine died in Return of the Jedi, he transferred his essence into an imperfect clone body.
Because of his new body’s hopelessly flawed composition, Palpatine’s cultists in the Sith Eternal worked tirelessly to create a better body for his essence to inhabit. The cultists tinkered with genes and muscle tissue, resulting in the creation of many abominations. Supreme Leader Snoke was one such abomination, but even though Snoke had power and a mind of his own his body was just as frail as Palpatine’s new one. As a result, Snoke was deemed a failure as a replacement body, but he was still useful to Palpatine’s plans with running the First Order and training Kylo Ren.
There was another clone of Palpatine that was physically healthy and stable, but had no Force-sensitivity. Because of the clone’s lack of power in the Force, Palpatine deemed him a failure and discarded him. In Sidious’s eyes, this clone’s only use was to further Sidious’s bloodline naturally. Sometime later, the clone had a wife or girlfriend and together they had Rey. Rey’s youth and immense power in the Force made her the ideal vessel for Palpatine’s essence to inhabit.
One could argue that Rey’s father is Palpatine’s “son” in the same sense that Boba Fett is the “son” of Jango Fett. Rey’s father also looks nothing like Palpatine despite being genetically identical. This is similar to the clone trooper unit known as the “Bad Batch.” Despite being clones of Jango Fett, the Bad Batch looked nothing like him.
Due to her father being a clone of Palpatine, one could argue that Palpatine was Rey’s biological father and grandfather at the same time. Because her father and Snoke were creations of Palpatine, Rey’s story was closely intertwined with Darth Sidious’s return. Ironically, Palpatine’s attempts to create a suitable body for himself ultimately led to his final demise.