* Annabeth Chase: This is the most likely answer as she's Percy's best friend and is practically considered family. However, they aren't biologically related.
* Thalia Grace: She was Percy's half-sister through their shared father, Poseidon. Unfortunately, she is often referred to as his *brother* in the books since she was turned into a tree.
* Bianca di Angelo: This is the sister of Nico di Angelo, who becomes a close friend and ally to Percy.
It's important to consider the context of the question. If you're looking for a biological sister, then Thalia Grace is the answer. But if you're looking for a close friend and ally who is considered a sister, then Annabeth Chase is a more fitting answer.