Well caleb is a nice guy really once u look past all the bad things he has done in his life i think his sister is pretty cute too (no homo) so its like why wouldn't he want to help her they grew up together it's only human
He cared for her until he chose Erudite as his faction,but as the saying goes "faction before blood" and he chose his faction before blood on meer than 1 occasion.In Insurgent,he didn't seem all that upset when Tris "died"(when Peter injected her with that serum) and he lied to her saying he was going back to Abnegation,when really he was going back to Erudite to Jeanine and inform her of Tris's plan.He is a traitor!!!!!!!!
I don't know what to think if he actually cares about his sister of not because he gave her over to erudite to die, he betrayed her. But also he was willing to give up his life for Tris and agreed to go to the room and stop it even when he knew would definitely die. It was a suicide mission. The reason he agreed to do it was because of guilt of how he betrayed his sister, the only family he has left.
