I'm making my brain hurt with this question.

I have a character, who's name is Robert. He has a step-brother, named Pearce. Pearce is the child of Issac and Alyssa. Isaac has a child by another woman. The child is named Rachel, and she's Pearce's half-sister. Alyssa divorced Isaac and married Robert's father. What is Rachel's relation to Robert, if any?

I came up with step-siblings once removed, but I know that's horribly wrong. Can someone figure out the correct terminology?

Thanks in advance.