Nathan hadn't thought much about going to help out, only knew he wanted to do so. While he typically only forged close connections with a few, Annie had managed to become part of that small circle in a brief time and he had no issue with opening that to include someone so important to Annie. Knowing he could actually help Annie's sister made him feel as though he could, in some small way, find another way to thank Annie. It wasn't an even trade, in helping each other's siblings, but it made him feel useful.
He knocked on the door frame when he arrived, catching sight of 'Sister' a moment later. "Hey, I said not to bother with anything else," he said, stepping into the room with a look on his face that wasn't, surprisingly, shocked. It was a look of clear worry, because he was processing the scene with a thrown together formula of Annie's previous acts he'd witnessed and her accompanying attitude at the time and ending up with a level of upset that left him very worried.
However, Annie was in the other room right now and her sister had a towel wrapped around her arm, so he needed to prioritize his concern. He moved to clear a spot to sit and then set the first aid supplies down before he beckoned her over.