They never discuss it; the time he died. It was longer than 12 minutes time, so technically she's the older of the two now, but they never discuss it, so it doesn't much matter. He still states that he's, "12 minutes older, you know," and she's just so grateful to have him back that she doesn't argue the point.
He doesn't know how long he was out for. He could've figured it out if he'd thought to look at a news report of the fight, but he didn't because he didn't care. He was there now, and he didn't want to think of the time he'd abandoned Wanda, leaving her alone like he never had before. His job was to protect his fragile twin from any pain; and then he'd gone and caused her more than she'd ever felt before. It was irresponsible of him, but then again she'd always been the more responsible of the two, thinking before she acted and all that. Wanda moved too slowly for him much of the time, but he'd never gone anywhere without her, not until that day.
It wasn't fair of him not to wait for her, that'd been her first thought once she'd calmed down enough to think rationally instead of just crying and killing. He'd always waited for her before, carrying her with him when she wasn't quick enough, which was nearly all the time. She'd always felt connected with him, even when they weren't physically touching she could feel him. He couldn't so he was always grabbing her hand, her wrist, her waist, her shoulder, something; anything to make sure she was still by his side, where she belonged. She blamed herself, of course; she'd sent him to fill the boats hadn't she? Against his better judgment he'd left her alone and then he'd died…that made it her fault. She didn't think she'd be able to live with the guilt, even after he'd 'come back' as they'd taken to calling it. Like he'd gone on a trip.
Wanda treated him differently after that. She wasn't as bothered with his need for perpetual assurance of her safety, by the constant physically contact he sought out, by his quick temper and frustration. While they used to have sibling tiffs, like any siblings do, now she just quietly accepted all the things about him that used to drive his much more grounded sister crazy and let him carry on as he would.
He tried harder and harder to rile her up, doing all the things that used to bother her, but she never responded in any way. He wanted things back to normal, and this was the only normal thing he could think of, in this wild and crazy new world of New York and The Avengers. It went on like this for some time, his poking and prodding at her, trying to garner some reaction from the girl, to find some piece of her that she'd apparently left behind in Sokovia, until the old man, the one they called Hawkeye, stopped him one day.
"Why are you torturing her like that?" he'd asked.
"You people, you broke my sister," he'd spat back angrily, "she's nothing like she used to be. It's like she's rolled over and given up!"
"We didn't break her," Barton barked. He'd come to be very protective of both the twins, but Wanda especially in the time since the big battle against Ultron, "When you saved me, when you died, you left her here. She was inconsolable for days. She was barely functional; didn't eat, didn't sleep, she just sat by your side while you were undergoing procedure after procedure. She wouldn't look anyone in the eye, she couldn't. Peitro, when you died, she died, but when you came back, she still didn't. You've only had to live 12 minutes without her, and you didn't know the difference then. She knew she was missing half of herself, half of her soul. So yes, she's been a little off lately, but you'll have to excuse her. Not everyone is as quick to recover as you are."
Pietro just looked back at the man, stunned and a bit confused. This stranger understood his sister better than he did, and in that moment he vowed to never let that happen again. So, that night, he tried to be good, to treat Wanda the way he should, so that she'd heal too. He found her curled up, asleep in a chair; she was never comfortable in these new Avenger's beds, there was too much room, too much space. She felt too alone.
"Wanda," he called from the doorway. He didn't want to invade her space and push her around like usual, he wanted her to act on her own.
"What is it Pietro?" she woke with a start at the sound of his voice, and he couldn't help but chuckle at himself, I am terrible at this.
"Come here," he stated quietly. She looked confused; she never went to him, he never waited for her to, he always just went to her.
"Yes?" she asked, approaching him. When she was within a foot away he closed the gap between them, but at a normal pace. It was unnerving to her, but she knew not what to do.
He wrapped his arms carefully around the delicate girl, and place a kiss on her forehead, whispering, "I am sorry sister."
"For what?" she asked.
"For what happened," he said by way of explanation.
She did not respond, so he continued, "For not considering what it would do to you and leaving you. Also for coming back and not considering what it had done to you. For everything."
She didn't answer and he released her so he could study her face for a reaction. She was crying. He hadn't seen her cry in the years since they were children.
He gripped her face in his hands and wiped away her tears with his thumbs, unsure of what else to do, until she started to laugh.
"I am so glad you are back with me brother," she said, burrowing her head into his chest and wrapping her thin arms around his narrow waist, "Do not go away from me again and I will not have reason to cry anymore."
That was a deal he was more than willing to make. From then on they were back to normal, but neither took the other for granted and both were much less alone.
He still had no idea what it was like for her when he'd gone, he just knew he never wanted to do that to her again.