Dean didn’t really notice, at first, that there was someone with Sam. Probably a stupid thing not to notice - why would Sam be bringing someone with him? Does he think he’ll need backup, does he seriously.... okay, it makes sense. Dean should have probably been more concerned with is Sam really Sam, honestly, but he wasn’t, he knew Sam was Sam - but if their positions were reversed he’d be suspicious, too.
When he did realize, he expected the vague shape of a head in the passenger seat to be some guy Sam knew and thought would be suitable backup if this went south, some hunter or something, or maybe that Castiel “angel”-dude who’d said he was coming over here, too. So when the person got out, and it was woman, he was a little confused for all of about ten seconds, before he recognized her, and then it felt like he couldn’t breathe.
Dean knew Mom was here - Sam said it was really her, and he’d kind of, in theory, come to terms with that. In theory, though, because in reality it was a little different, was a lot like being hit over the head with something heavy, because the last time he’d seen her he was four years old and then she’d died. Dad had died too, but while he missed his father and he was really hoping the man beside him was John Winchester, it was... different, the grief for Dad and the grief for Mom, and, hell, even the grief for Sam was different from that. Mom was a missing piece he’d lost so early on that it had gotten harder to remember her - her voice, her smile, her face - while Dad stuck in his memory a lot clearer, and Sam had come back before he’d had time to start to forget.
He was distantly aware that his brother was standing there, distantly aware of Dad or the thing that looked like him, at his side, but he was sort of staring at Mom, instead of paying either of them much attention at the moment, “Mom?” breathed quiet enough he didn’t think she’d be able to hear, and he took a shaky step forward and then couldn’t take another, frozen where he was.