Step siblings (Percy Jackson story)

"I called Your sister Percy " annabeth said "wait what I have a sister" Percy asked Compleatly confused then they heard a thump
"Step sister but we used to be very close" Percy turned around and saw that a girl about thirteen had jumped on the Argo 2 and it clicked he remembered his step sister
Lindsey
What happens when Percy gets a step sister and she spends almost six months looking for him
find out in the Lindsey Jonson trilogy book 1 step siblings