An Extremely Rare Disease
For some reason, Taylor would always ask her mother if she had a long-lost sibling or twin that she'd never met, but the truth was that she had only given birth to one baby. One day, she was watching a documentary about a rare medical condition known as chimerism, a disease where the body's cells have more than one distinct genotype, and she realized that the patient that was being interviewed had her same skin patterns.
Only 100 cases of chimerism had been documented in history, so it was highly unlikely she suffered from it too. But when Taylor showed her strange birthmarks to her doctor, they made her undergo a few tests, and the results proved she did have chimerism after all! This explained why she had always wanted a twin as well as why everything on one side of her body were a bit larger.