Cas was torn again, on one hand he was relieved to have avoided the potential fight with his little sister, but on the other he had wanted Jenny to do this herself, to actually comply with his wishes. The little voice in his head pointed out that the only way Jenny was going to do that would be with a sore bottom and he was supposed to be trying to avoid that. He couldn’t argue with that, so he forced his face into a smile as well, "thanks Sirius."