Nathan spared a glance at Peter at his revelation. Oh shit. Part of him braced for his mother to react even worse if she thought this was some twisted placed in her head, now knowing that, but there was still concern for Peter, as Nathan hadn't missed that flinch in his peripheral vision. But Peter had seen the result of that event, had healed it himself, and their mother didn't have the advantage of that knowledge to make this real for her.
He was already trying to prepare an explanation when she moved forward, but he didn't advance, simply stood there and let her discover he wasn't on the brink of death for herself. He didn't move at all until she clung to him, her words confirming she either no longer thought this a nightmare in her own head or at least couldn't resist it. The later thought, that there was a very real possibility she could still think this was a trick of her mind, pained him and he wrapped his arms around her in return, holding her tightly. Right now, at least for the moment, what had happened in the past didn't matter, just as it hadn't the last time she'd been so terrified and nearly ended up dead. He couldn't stand to see her like this, even now.
"I'm all right, Ma, healed better than new, and you're all right, nothing's gonna get you now," he said as he tightened his arms, striving for some security for her and even trying to make it seem real just by force of embrace if necessary. "I know it's confusing, but I swear to you, it's all real. It's all real and we're all just fine."
He slanted another look at Peter and it was only then it hit him why she hadn't latched on to Peter first, the reaction he would have expected from his mother when faced with both of them at once. It was because his brother didn't look like Peter Petrelli right now. It wasn't that Nathan had entirely forgotten the altered appearance, as at first it had been strange, but having Peter's mental voice still the same had let him set that strangeness aside.
You want in on this? Even his mental voice was soft and understanding, as Nathan wasn't going to push this. If Peter wanted his own clinging embrace, he'd make their mother understand what was going on, and if Peter wanted to remain anonymous for now, Nathan would cover for him.