Nope, Kaitlin Olson is not related to the Olsen twins—not even a little bit. It’s a super common mix-up, probably because their last names sound almost identical (Olson vs. Olsen), and they’re all actresses. But they come from totally different families and backgrounds.
Kaitlin is best known for playing Dee on It’s Always Sunny in Philadelphia, and she was born in Oregon. Meanwhile, Mary-Kate and Ashley Olsen are twins who started acting as babies on Full House, and they’re from California. Also, the spelling of their last names is different—Olson with an “O” for Kaitlin, and Olsen with an “E” for the twins. So yeah, despite the name similarity, there’s zero relation between them.