: I did not create and do not own these characters. Joss Whedon created BtVS, Anthony Zuiker created CSI. I'm playing with other people's toys.
1. He thought Buffy was hot. Not that he needed to tell Catherine Buffy was hot- she wasn’t blind. But he was pretty sure Catherine would cut him up into tiny little pieces that would never be found if he even thought of touching her sister. Sam Braun would probably help her, albeit for slightly different reasons. But there was another reason not to say anything: it was so obvious that Buffy and Nick were made for each other. No way he was going to screw up what could be the best thing ever to happen to his best friend.
2. Dawn had been kidnapped at least 3 times other than that time in the Rampart. He was pretty sure that fell into the category of Things Catherine Can’t Handle. He had promised Dawn that time that anything she told him was strictly confidential, unless it was life or death. Kidnappings that were long done and over with weren’t life or death. Two of those other kidnappings she told him about right away. She treated them like a joke. The third time he didn’t hear about until after Nick’s abduction, when Sara told him Dawn needed to talk to him.
3. Buffy died twice. Catherine knew about the first time. But the second one- that one had freaked him out. He’d sat up with Dawn the rest of that miserable night waiting for Buffy to snap out of her flashbacks and come home. After what Dawn had confessed about the night she got her scars, he demanded the truth from Buffy. When she’d told him, he kind of wished she hadn’t. But in a way, he was glad. This was clearly something she’d needed to talk out for a while now. He figured she’d tell Catherine when she was ready.
4. Dawn slept with Greg on her 18th birthday. He hadn’t told Buffy either. Hell, if he could have wiped it from his memory, he would have. That was not something he had wanted to walk in on. It was the first time he’d wished he could bleach his brain. The next day, he’d torn strips out of a mortified Greg for being so stupid. Later that evening, he’d thoroughly embarrassed Dawn with a big brotherly lecture about being careful, which included not only appropriate prophylactic measures against pregnancy and STDs, but also picking locations more carefully and locking doors
5. Buffy had told him last week that the anagogic demon who worked at the Tropicana overheard Catherine humming and said that she was going to lose someone important to her next week. Buffy had been in tears. She was sure it meant her number was up. He’d tried to reassure Buffy nothing was going to happen to her. But he’d promised that if anything did happen, he’d make sure Cath, Dawn, Lindsey, and Nick were ok. Buffy cared about her family more than anything else in the world. He’d have told Cath about that if he’d gotten the chance.