He really wanted nothing more than to believe Alastair when he said that, that he loved him and all of his feelings were real, regardless of what actually brought them back together. He wanted it more than anything. But Charlie couldn't. There was a doubt, a very tiny one creeping up from the back of his mind. A doubt that was able to take form because Charlie realized he couldn't tell when he was being lied to. The man in front of him had been physically beaten and appeared broken and devastated and Charlie hadn't been able to tell it was an act.
How was he supposed to trust that this was real? He absolutely believed that Alastair had done all of this for his sister, because he knew the other man would do anything to protect her, this was just more proof of that. So he genuinely didn't think that any of this had happened because Alastair's goal had been to aid the Death Eaters. But even if he did want to protect his sister more than anything, and didn't actually want the Death Eaters in power, that didn't mean anything between them personally was real. And Charlie hated that he had to question it.
"Is Delilah the reason you're here?" There was no judgment or condemnation in his tone--Charlie would understand if understand if the answer was yes--but there was still emotion there as he developed a sinking feeling in his stomach, dreading the answer.